The support for sending alerts via Email or SNMP was quite a kludge,
and there are much better external tools for managing alerts than
what we can supply in core anyways so this retires the capability.

All references to alert sending in the docs are removed, but there
needs to be section written about how to migrate off this feature
in the release notes or a similar location.

Discussion: https://github.com/greenplum-db/gpdb/pull/6384