patch 8.0.0241: fallback implementation of mch_memmove is unused

Problem:    Vim defines a mch_memmove() function but it doesn't work, thus is
            always unused.
Solution:   Remove the mch_memmove implementation. (suggested by Dominique
            Pelle)
diff --git a/src/os_unix.h b/src/os_unix.h
index d28aa4d..695affa 100644
--- a/src/os_unix.h
+++ b/src/os_unix.h
@@ -423,21 +423,17 @@
 # endif
 #endif
 
-/* memmove is not present on all systems, use memmove, bcopy, memcpy or our
- * own version */
-/* Some systems have (void *) arguments, some (char *). If we use (char *) it
+/* memmove() is not present on all systems, use memmove, bcopy or memcpy.
+ * Some systems have (void *) arguments, some (char *). If we use (char *) it
  * works for all */
-#ifdef USEMEMMOVE
+#if defined(USEMEMMOVE) || (!defined(USEBCOPY) && !defined(USEMEMCPY))
 # define mch_memmove(to, from, len) memmove((char *)(to), (char *)(from), len)
 #else
 # ifdef USEBCOPY
 #  define mch_memmove(to, from, len) bcopy((char *)(from), (char *)(to), len)
 # else
-#  ifdef USEMEMCPY
+    /* ifdef USEMEMCPY */
 #   define mch_memmove(to, from, len) memcpy((char *)(to), (char *)(from), len)
-#  else
-#   define VIM_MEMMOVE	    /* found in misc2.c */
-#  endif
 # endif
 #endif