Core: Drop custom copymem/zeromem defines

We've been using standard C library functions `memcpy`/`memset` for these since
2016 with 67f65f6639.

There was still the possibility for third-party platform ports to override the
definitions with a custom header, but this doesn't seem useful anymore.

core/io/zip_io.cpp

@@ -30,8 +30,6 @@
 
 #include "zip_io.h"
 
-#include "core/os/copymem.h"
-
 void *zipio_open(void *data, const char *p_fname, int mode) {
 	FileAccess *&f = *(FileAccess **)data;
 
@@ -103,7 +101,7 @@ int zipio_testerror(voidpf opaque, voidpf stream) {
 
 voidpf zipio_alloc(voidpf opaque, uInt items, uInt size) {
 	voidpf ptr = memalloc(items * size);
-	zeromem(ptr, items * size);
+	memset(ptr, 0, items * size);
 	return ptr;
 }