Relax BCR presubmit config block check. (#2718)

bcr_presubmit: This commit reverts "Require maintainer review of risky
config changes. (#2685)" (36dd157dc727d2787c4f8f515825e63bdb52e4be)
since existing checks are good enough.

bazelci.py: Only block if BCR's own presubmit.yml file is being changed.