CI: remove unused library

[casperdcl]

As per #199 (comment) it seems the static lib is a CMake build hack which isn't actually necessary nor desired for distribution in a header-only package.

This PR:

• reduces package size from 17MB to 0.5MB (tar.gz from 1.8MB to 0.066MB)
• tests that cpp/examples still builds sans libpetsird.a

[KrisThielemans]

This library is generated by

PETSIRD/cpp/CMakeLists.txt

Lines 40 to 44 in ddbcf12

	# petsird_generated is an OBJECT library. Those are a bit hard
	# to handle for external projects, so create a "real" library as well.
	# This is the one people should use.
	add_library(petsird STATIC)
	target_link_libraries(petsird PUBLIC petsird_generated)

It is referenced

(petsird2) kris@KrisXPS:~/miniforge3/envs/petsird/lib/cmake/PETSIRD-0.9$ grep -r libpets
PETSIRDTargets-debug.cmake:  IMPORTED_LOCATION_DEBUG "${_IMPORT_PREFIX}/lib/libpetsird.a"
PETSIRDTargets-debug.cmake:list(APPEND _cmake_import_check_files_for_PETSIRD::petsird "${_IMPORT_PREFIX}/lib/libpetsird.a" )

I've created #208. This works locally (let's see what CI says). It does fail when I remove libpetsird.a.

[KrisThielemans]

If you can change the CMakeLists.txt to avoid creating the .a (I couldn't), feel free, but I have no headspace for this anymore and won't be able to review. I strongly suggest to close and got onto more urgent stuff.