Remove repmgr from Omnibus

This is a separate issue to remove repgmr that is to be done before PostgreSQL 11 is removed.

The following discussion from !5261 (merged) should be addressed:

• @twk3 started a discussion:

  @rmarshall can you open a new issue and MR for the repmgr removal, and lets go ahead and tackle that right away prior to merging this MR.

  We need:

  • The software removal of rempgr you did here
  • Removing the repmgr cookbook from our source code
  • Remove any other repmgr work from our roles
  • Keep the repmgr attributes in https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/files/gitlab-cookbooks/package/libraries/config/gitlab.rb but add a deprecation notice for when its not set to false
    • an example of the deprecation notice is how we did unicorn in https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/files/gitlab-cookbooks/package/libraries/deprecations.rb