Remove fork of MapBufferBuilder.cpp

[rectified95]

Build break coming from Core. PR to fix this: facebook/react-native#31363

[chrisglein]

Is there a way we can get coverage for different warning levels on the nightly builds to catch this more in advance?

[rectified95]

This should have been removed in #7680
Can be deleted any time now. Will most likely in the next ingestion from core.