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.

drivers/png/image_loader_png.cpp

@@ -88,7 +88,7 @@ Vector<uint8_t> ImageLoaderPNG::lossless_pack_png(const Ref<Image> &p_image) {
 	{
 		// must be closed before call to image_to_png
 		uint8_t *writer = out_buffer.ptrw();
-		copymem(writer, "PNG ", 4);
+		memcpy(writer, "PNG ", 4);
 	}
 
 	Error err = PNGDriverCommon::image_to_png(p_image, out_buffer);