Drop the unused LLVM download from setup-qnn-deps.sh (#24098)

## Problem

`.ci/scripts/setup-qnn-deps.sh` calls `setup_libcpp`, which downloads a
575 MB LLVM release from GitHub. A connection reset in the middle of
that download fails the job:

```
curl: (56) OpenSSL SSL_read: Connection reset by peer, errno 104
Error: Failed to download LLVM
```

But nothing uses what `setup_libcpp` installs:

- Every workflow runs `setup-qnn-deps.sh` with `bash`, not `source`. So
the `LD_LIBRARY_PATH`, `LIBRARY_PATH` and `CPLUS_INCLUDE_PATH` values it
exports are gone when the script exits, and nothing else reads its
install folder.
- Its copy step does not pick up libc++ at all. The libc++ libraries are
in a subfolder its `lib/*.so*` glob does not reach.
- The QNN backend test jobs never call it, and they pass. The CI image
gets libc++ from the system package.

## Fix

Remove the `setup_libcpp 12` call and the `setup_libcpp` function, which
nothing else uses. This fixes the flaky download and saves about half a
minute of download and extraction per QNN job.

The Python setup path (`_stage_libcxx` in `download_qnn_sdk.py`)
downloads the same archive for users who have not set `QNN_SDK_ROOT`. It
is separate and not changed here.

## Testing

Shell syntax check on both scripts. The QNN CI jobs on this pull request
are the real test, since they run `setup-qnn-deps.sh` without the
download.


cc @cbilgin @psiddh

Author: shoumikhin

.ci/scripts/setup-qnn-deps.sh

@@ -9,7 +9,6 @@ set -ex
 
 source "$(dirname "${BASH_SOURCE[0]}")/../../backends/qualcomm/scripts/install_qnn_sdk.sh"
 
-setup_libcpp 12
 setup_android_ndk
 install_qnn
 pip install -r backends/qualcomm/requirements.txt

backends/qualcomm/scripts/install_qnn_sdk.sh

@@ -203,68 +203,3 @@ install_hexagon_sdk() {
   echo "Set HEXAGON_SDK_ROOT=${HEXAGON_SDK_ROOT}"
   echo "Set HEXAGON_TOOLS_ROOT=${HEXAGON_TOOLS_ROOT}"
 }
-
-setup_libcpp() {
-  clang_version=$1
-  LLVM_VERSION="14.0.0"
-  INSTALL_DIR="/tmp/libcxx-${LLVM_VERSION}"
-
-  # Check if we already have a local installation
-  if [ -d "${INSTALL_DIR}/include" ] && [ -d "${INSTALL_DIR}/lib" ]; then
-    echo "Local libc++ already installed at ${INSTALL_DIR} - skipping"
-    # Set environment variables
-    export CPLUS_INCLUDE_PATH="${INSTALL_DIR}/include:$CPLUS_INCLUDE_PATH"
-    export LD_LIBRARY_PATH="${INSTALL_DIR}/lib:$LD_LIBRARY_PATH"
-    export LIBRARY_PATH="${INSTALL_DIR}/lib:$LIBRARY_PATH"
-    return
-  fi
-
-  echo "Installing libc++ manually to ${INSTALL_DIR}"
-
-  # Create temporary directory
-  TEMP_DIR=$(mktemp -d)
-  # Ensure cleanup on exit or return
-  trap 'rm -rf "$TEMP_DIR"' RETURN
-
-  pushd "${TEMP_DIR}" >/dev/null
-
-  BASE_NAME="clang+llvm-${LLVM_VERSION}-x86_64-linux-gnu-ubuntu-18.04"
-  LLVM_URL="https://github.com/llvm/llvm-project/releases/download/llvmorg-${LLVM_VERSION}/${BASE_NAME}.tar.xz"
-
-  echo "Downloading LLVM from ${LLVM_URL}"
-  curl --retry 3 -fLO "${LLVM_URL}" || {
-      echo "Error: Failed to download LLVM"
-      exit 1
-  }
-
-  echo "Extracting ${BASE_NAME}.tar.xz"
-  tar -xf "${BASE_NAME}.tar.xz" || {
-      echo "Error: Failed to extract LLVM archive"
-      exit 1
-  }
-
-  # Create installation directory
-  mkdir -p "${INSTALL_DIR}/include"
-  mkdir -p "${INSTALL_DIR}/lib"
-
-  # Copy libc++ headers and libraries
-  cp -r "${BASE_NAME}/include/c++/v1/"* "${INSTALL_DIR}/include/"
-  cp -r "${BASE_NAME}/lib/"*.so* "${INSTALL_DIR}/lib/"
-
-  popd >/dev/null
-
-  # Create necessary symlinks locally
-  pushd "${INSTALL_DIR}/lib" >/dev/null
-  ln -sf libc++.so.1.0 libc++.so.1
-  ln -sf libc++.so.1 libc++.so
-  ln -sf libc++abi.so.1.0 libc++abi.so.1
-  ln -sf libc++abi.so.1 libc++abi.so
-  popd >/dev/null
-
-  # Set environment variables
-  export CPLUS_INCLUDE_PATH="${INSTALL_DIR}/include:${CPLUS_INCLUDE_PATH:-}"
-  export LD_LIBRARY_PATH="${INSTALL_DIR}/lib:${LD_LIBRARY_PATH:-}"
-  export LIBRARY_PATH="${INSTALL_DIR}/lib:${LIBRARY_PATH:-}"
-
-  echo "libc++ installed to ${INSTALL_DIR}"
-}