bot: remove old branch config from mergify
bot: remove old branch config from mergify

We don't backport to 1.4 or 1.5 anymore so let's clean up the file.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

rook/rook,rook/rook,rook/rook,rook/rook,rook/rook