[9.11] chg: Remove danger checks for release notes and CHANGES

Since 9.21.0-dev, the release notes and changelog process has been
changed. Backport to the EoL branch are no longer expected to have
either CHANGES or release notes, as we aren't going to release any more
versions anyway.

Related #75

Merge branch '75-gitchangelog-9.11' into 'bind-9.11'

See merge request isc-projects/bind9!9183