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/http_client.cpp

@@ -633,7 +633,7 @@ PackedByteArray HTTPClient::read_response_body_chunk() {
 
 					ret.resize(chunk.size() - 2);
 					uint8_t *w = ret.ptrw();
-					copymem(w, chunk.ptr(), chunk.size() - 2);
+					memcpy(w, chunk.ptr(), chunk.size() - 2);
 					chunk.clear();
 				}