Remove fork handling from release issue workflow (#79310)

This is currently broken, because the check is performed on the wrong
repository. repo here is llvm/llvm-project, which is not a fork (so this
will always trigger), then we'll push a new branch to
llvmbot/llvm-project, and then again set the wrong owner, so we'll look
for the branch in llvm/llvm-project rather than llvmbot/llvm-project.

Rather than fixing this, I'm removing the code entirely, as it shouldn't
be needed anymore (llvmbot/llvm-project is a fork of llvm/llvm-project).