Description of the problem, including code/CLI snippet

Merge request approval rules were added in #1200 which implemented list, create, and update, but delete was not included for some reason. Gitlab API supports deleting these rules, so we should implement deletion for these events as well, such as

approval_rules = project.approval_rules.list()
approval_rules[0].delete()

Expected Behavior

Approval rules can be deleted.

Actual Behavior

The delete function does not exist.

Specifications

• python-gitlab version: 2.9.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 13.12EE (the delete API has existed since at least v11)