[llvm][docs] Remove guidance on adding release:reviewed label (#164395)
"How To Release LLVM To The Public" [1] mentions to add the
release:reviewed label once a bug has been reviewed, but looking at the
label [2] it seems this hasn't been followed for quite a long time, so I
propose we remove it.
[1]
https://llvm.org/docs/HowToReleaseLLVM.html#triaging-bug-reports-for-releases
[2]
https://github.com/llvm/llvm-project/issues?q=label%3Arelease%3Areviewed