[CI] Remove old premerge bots

This patch removes the old LLVM premerge bots from llvm-zorg in preparation
for adding in the new ones.

Reviewers: cmtice, dschuff, gkistanova, gburgessiv, Keenuts, lnihlen

Reviewed By: cmtice, Keenuts

Pull Request: https://github.com/llvm/llvm-zorg/pull/529