[libc][fenv] Remove unnecessary dependencies (#88788)

Remove the fenv macro dependency from the CMake files as the underlying targets
do not make use of it. Note that we do not have to worry about
[corresponding Bazel targets](https://github.com/llvm/llvm-project/blob/main/utils/bazel/llvm-project-overlay/libc/BUILD.bazel#L1138-L1288),
as they look good.