memory-util: drop unused memdup_reverse()

With the previous commit, now the function is unused anymore.
Let's drop it.

src/basic/memory-util.h

@@ -98,6 +98,3 @@ static inline void erase_and_freep(void *p) {
 static inline void erase_char(char *p) {
         explicit_bzero_safe(p, sizeof(char));
 }
-
-/* Makes a copy of the buffer with reversed order of bytes */
-void* memdup_reverse(const void *mem, size_t size);