Core: Drop custom copymem/zeromem defines

We've been using standard C library functions `memcpy`/`memset` for these since
2016 with 67f65f66391327b2967a20a89c3627e1dd6e84eb.

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.

platform/javascript/http_client_javascript.cpp

@@ -209,7 +209,7 @@ PackedByteArray HTTPClient::read_response_body_chunk() {
 		return chunk;
 	}
 	chunk.resize(read);
-	copymem(chunk.ptrw(), response_buffer.ptr(), read);
+	memcpy(chunk.ptrw(), response_buffer.ptr(), read);
 	return chunk;
 }