Remove deprecated "master" role

What does this MR do?

A follow up to https://gitlab.com/gitlab-org/gitlab-ce/issues/42751 to remove deprecations in the backend related to the term "master role" which was replaced by "maintainer role".

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.