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/math/transform.cpp

@@ -31,7 +31,6 @@
 #include "transform.h"
 
 #include "core/math/math_funcs.h"
-#include "core/os/copymem.h"
 #include "core/string/print_string.h"
 
 void Transform::affine_invert() {