retention: Clean up redundant code for special handling of UserMessages.
retention: Clean up redundant code for special handling of UserMessages.

UserMessages no longer need special handling, they can be archived by
move_models_with_message_key_to_archive and automatically cleaned up
like the other models with a message key with CASCADING=True.

tommyip/zulip,andersk/zulip,zulip/zulip,kou/zulip,kou/zulip,timabbott/zulip,showell/zulip,rht/zulip,tommyip/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,synicalsyntax/zulip,brainwane/zulip,tommyip/zulip,rishig/zulip,tommyip/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,tommyip/zulip,synicalsyntax/zulip,punchagan/zulip,rht/zulip,brainwane/zulip,punchagan/zulip,brainwane/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,rishig/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,rht/zulip,kou/zulip,tommyip/zulip,zulip/zulip,synicalsyntax/zulip,brainwane/zulip,zulip/zulip,andersk/zulip,shubhamdhama/zulip,punchagan/zulip,timabbott/zulip,rishig/zulip,kou/zulip,showell/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,rht/zulip,tommyip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,eeshangarg/zulip,timabbott/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,eeshangarg/zulip,timabbott/zulip,timabbott/zulip,showell/zulip,rishig/zulip,punchagan/zulip,brainwane/zulip,rishig/zulip,kou/zulip,timabbott/zulip,hackerkid/zulip,zulip/zulip,synicalsyntax/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rishig/zulip,shubhamdhama/zulip,timabbott/zulip,zulip/zulip,andersk/zulip,rishig/zulip,eeshangarg/zulip