[libc] Remove `#ifdef __cplusplus` part from `include/llvm-libc-macros/math-macros.h`. (#87864)

Now with the proxy header `hdr/math_macros.h`, the header
`include/llvm-libc-macros/math-macros.h` will not be included in overlay
mode, and the extra definitions for `__cplusplus` mode is not needed any
more.