[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.

Author: lntue

libc/include/llvm-libc-macros/math-macros.h

@@ -51,33 +51,9 @@
 #define math_errhandling (MATH_ERRNO | MATH_ERREXCEPT)
 #endif
 
-// These must be type-generic functions.  The C standard specifies them as
-// being macros rather than functions, in fact.  However, in C++ it's important
-// that there be function declarations that don't interfere with other uses of
-// the identifier, even in places with parentheses where a function-like macro
-// will be expanded (such as a function declaration in a C++ namespace).
-
 // TODO: Move generic functional math macros to a separate header file.
-#ifdef __cplusplus
-
-template <typename T> inline constexpr bool isfinite(T x) {
-  return __builtin_isfinite(x);
-}
-
-template <typename T> inline constexpr bool isinf(T x) {
-  return __builtin_isinf(x);
-}
-
-template <typename T> inline constexpr bool isnan(T x) {
-  return __builtin_isnan(x);
-}
-
-#else
-
 #define isfinite(x) __builtin_isfinite(x)
 #define isinf(x) __builtin_isinf(x)
 #define isnan(x) __builtin_isnan(x)
 
-#endif
-
 #endif // LLVM_LIBC_MACROS_MATH_MACROS_H