moveonly: move IsSelectableSocket() from compat.h to sock.{h,cpp}

To be converted to a method of the `Sock` class.

src/compat/compat.h

@@ -109,14 +109,6 @@ typedef char* sockopt_arg_type;
 #define USE_POLL
 #endif
 
-bool static inline IsSelectableSocket(const SOCKET& s) {
-#if defined(USE_POLL) || defined(WIN32)
-    return true;
-#else
-    return (s < FD_SETSIZE);
-#endif
-}
-
 // MSG_NOSIGNAL is not available on some platforms, if it doesn't exist define it as 0
 #if !defined(MSG_NOSIGNAL)
 #define MSG_NOSIGNAL 0