Add `Mutation.memberRoleDelete` to delete a member role

What does this MR do and why?

This change adds a new GraphQL Mutation that can be used to delete a MemberRole.

#429885 (closed)

DELETE FROM "member_roles"
WHERE "member_roles"."id" = 1;

Time: 3.422 ms
  - planning: 0.451 ms
  - execution: 2.971 ms
    - I/O read: 2.878 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 3 (~24.00 KiB) from the buffer pool
  - reads: 1 (~8.00 KiB) from the OS file cache, including disk I/O
  - dirtied: 0
  - writes: 0

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23765/commands/76256

Screenshots or screen recordings

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