SweepRule: remove obsolete git attributes file

SweepRule/attributes

-# git attributes for manual merges of production branches into master
-#
-# Requires the following merge driver to be defined:
-#   git config --global merge.ours.driver true
-#
-# See https://atlassoftwaredocs.web.cern.ch/guides/release_coordinators
-#
-
-# Ignore changes to Projects and build scripts:
-Projects/** merge=ours
-Build/** merge=ours