MSYS Makefiles: Drop outdated enforcement of archiver availability

Since this generator was added, it enforced availability of `CMAKE_AR`
during `enable_language`.  These days, the ABI detection and/or check
for a working compiler have already been run via `try_compile`, so if
the archiver is required but missing we will have already failed.  Drop
the early generator-specific enforcement that `CMAKE_AR` has been found.
It's needed only if the project actually creates an archive.