Add missing delete method in mattermost session

What does this MR do?

This MR fix an issue when we created a group with the setting 'Create a Mattermost team for this group' turned on and later tried to remove it.

The error was:

NoMethodError: undefined method `delete' for #<Mattermost::Session:0x00008fa05b072a00>
 from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/client.rb:28:in `delete'
    from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/client.rb:45:in `block in session_delete'
    from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/session.rb:42:in `block in with_session'
    from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/session.rb:145:in `with_lease'
    from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/session.rb:38:in `with_session'
    from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/client.rb:12:in `with_session'
    from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/client.rb:44:in `session_delete'
    from /opt/gitlab/embedded/service/gitlab-rails/lib/mattermost/team.rb:22:in `destroy'
    from /opt/gitlab/embedded/service/gitlab-rails/app/models/chat_team.rb:8:in `remove_mattermost_team'
    from (irb):6
    from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.8/lib/rails/commands/console.rb:110:in `start'
    from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.8/lib/rails/commands/console.rb:9:in `start'
    from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:68:in `console'
    from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.8/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
    from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.8/lib/rails/commands.rb:17:in `<top (required)>'
    from bin/rails:9:in `require'
    from bin/rails:9:in `<main>'

⚠ **Warning:**
This MR fix the issue in the gitlab side but a bug appears in mattermost when the API to delete a team is called see open github issue and the open Jira issue
So we need to wait a mattermost fix before merge

Why was this MR needed?

To be able to delete a group created with the setting 'Create a Mattermost team for this group' turned on.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Unit test
• Test with Mattermost 4.7
• Review
  • Has been reviewed by Backend
• Squashed related commits together

What are the relevant issue numbers?

Resolve #37367 (closed) and resolve #40660 (closed)
Related to omnibus-gitlab!2313 (merged)