Remove unused gitlab:geo:check_database_replication_working rake task

What does this MR do and why?

Removes the gitlab:geo:check_database_replication_working rake task as it is no longer used.

Relates to #495128

Changelog: removed

EE: true

How to set up and validate locally

1. Pull down this branch and attempt to run the following from the terminal.

   bundle exec rake gitlab:geo:check_database_replication_working

2. The command should fail as the task above has been removed.

Related to #495128