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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

When that happens, the [entire](https://github.com/llvm/llvm-project/blob/24661001281da09aa6c5c12e4f2552c9d06a8e88/flang/tools/f18/CMakeLists.txt#L141) [code](https://github.com/llvm/llvm-project/blob/24661001281da09aa6c5c12e4f2552c9d06a8e88/flang/tools/f18/CMakeLists.txt#L172) for omp_lib.mod generation in LLVM_ENABLE_PROJECTS=openmp builds should be removed.

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