Delete syncbranches script/workflow (#79245)
As majority of the changes are going thru @pytorchmergebot and
there is a good enough tooling for making sure that code-development
changes are merged as well
https://github.com/pytorch/pytorch/tree/fbsync branch will still stay in
place can be use to verify if something is amiss
Fixes #ISSUE_NUMBER
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79245
Approved by: https://github.com/seemethere
