chore: remove partial-release.txt after the LTS 11 release (#14927)

Follow-up to #14924. The LTS 11 partial release from `1.90.x` is
complete: `grpc-gcp` 1.14.1 and the `java-spanner` 6.121.1 modules are
on Maven Central.

- This PR removes `partial-release.txt` from `1.90.x`.
- If the file stays, each later tag on this branch is also a partial
release that builds only `grpc-gcp-java` and `java-spanner`.
- No code or version changes.

Author: hj690