Remove `OPENMP_ENABLE_LIBOMPTARGET` CMake option

[Flamefire]

It seems that after #75125 this option has no effect and is only passed to ENABLE_LIBOMPTARGET define in a header: https://github.com/llvm/llvm-project/blob/aa71344acdb1a861c1d74345a3fc80aef48bb4b3/openmp/runtime/src/kmp_config.h.cmake#L100C9-L100C28

It doesn't seem to make sense to make this define depend on a possibly manually specified variable instead of whether offload is actually a built runtime. At least it seems like a potential source of inconsistency