[llvm] [Flang][LLVM] Remove leftover CMake for old flang handling (PR #137049)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> * [[Flang] Add omp_lib dependency to check-flang #130975](https://github.com/llvm/llvm-project/pull/130975) is for the dependencies for the LLVM_ENABLE_RUNTIMES=openmp build
> 
>     * This code deleted in this PR is for ensuring dependencies in the LLVM_ENABLE_PROJECTS=openmp build
> 
> 
> Unless you entirely remove support for `LLVM_ENABLE_PROJECTS=openmp`, this code in this PR is still needeed.

Well, it's been made a warning and will probably be an error by the time the release rolls out. But I guess we can delay this if needed.

https://github.com/llvm/llvm-project/pull/137049