[clang-tidy] Enable plugin tests with LLVM_INSTALL_TOOLCHAIN_ONLY (#90370)

The only reason for the removed condition was that there was a
dependency for `CTTestTidyModule` on the `clang-tidy-headers` target,
which was only created under the same `NOT LLVM_INSTALL_TOOLCHAIN_ONLY`
condition. It looks like this target is not needed for
`CTTestTidyModule` to build and run, so the dependency can be removed
along with the condition.

See also https://reviews.llvm.org/D111100 for earlier discussions.