MDEV-15250 fixup: Remove MY_GNUC_PREREQ

The macro MY_GNUC_PREREQ() was used for testing for some minor
GCC 4 versions before GCC 4.8.5, which is the oldest version
that supports C++11, which we depend on ever since
commit d9613b750c

include/my_compiler.h

@@ -77,10 +77,6 @@
   Generic (compiler-independent) features.
 */
 
-#ifndef MY_GNUC_PREREQ
-# define MY_GNUC_PREREQ(maj, min) (0)
-#endif
-
 #ifndef MY_ALIGNOF
 # ifdef __cplusplus
     template<typename type> struct my_alignof_helper { char m1; type m2; };