Drop mail_room as a separate dependency

We introduced mail_room as a separate dependency because updating
mail_room required an updated net-imap gem. This gem attempted to
install the digest gem, but due to a bug present in Ruby 2.7
(https://bugs.ruby-lang.org/issues/17761), the installation failed and
caused Cloud Native GitLab builds to fail
(https://gitlab.com/gitlab-com/gl-infra/production/-/issues/4053).

We've updated to Ruby 3.0.5 for Cloud Native GitLab images now and
have already bumped the net-imap dependency in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/115755. With
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/116494, we've
now updated GitLab Rails to use mail_room v0.0.23. As a result, we no
longer need a separate dependency for Omnibus.

Removing this dependency makes it easier to update mail_room since we
only need to bump the `Gemfile` in GitLab Rails.

Changelog: changed

config/projects/gitlab.rb

@@ -112,7 +112,6 @@ dependency 'runit'
 dependency 'go-crond'
 dependency 'docker-distribution-pruner'
-dependency 'mail_room'
 dependency 'grafana-dashboards'
 if Build::Check.include_ee?
   dependency 'consul'

config/software/mail_room.rb

-#
-# Copyright:: Copyright (c) 2021 GitLab Inc.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-#
-
-name 'mail_room'
-default_version '0.0.23'
-
-license 'MIT'
-license_file 'LICENSE.txt'
-
-skip_transitive_dependency_licensing true
-
-dependency 'ruby'
-
-build do
-  patch source: 'add-license-file.patch'
-
-  env = with_standard_compiler_flags(with_embedded_path)
-  gem "install gitlab-mail_room --no-document --version #{version}", env: env
-end