Remove old docs regarding the "maintain" role

[ehuss]

This removes the old docs for the "maintain" role and how it relates to using bors. There are multiple reasons:

• Since switching to rulesets, users with "maintain" role can no longer override the protections. Rulesets behave differently than branch protections, and require explicitly listing who can bypass the rulset.
• This was only really relevant when using bors, and we generally only have one repository now that does that (rust-lang/rust), so it is not relevant to most users.
• The way it was written was confusing, since it implied users with "write" couldn't merge. What it meant to say is that users with "maintain" can override the protections (which per the first point is no longer true anyways).

[jieyouxu]

Thanks

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github
[INFO  rust_team::sync] 💻 Team Diffs:
    📝 Editing team 'rust-lang/project-assumptions-on-binders':
      Deleting member 'rust-lang-owner'
    💻 Repo Diffs:
    📝 Editing repo 'rust-lang/project-assumptions-on-binders':
      Environments:
        🔄 Update: github-pages
            Removing branches: main

[ehuss]

@jieyouxu Does it make sense why this PR triggered so many edits?

 rust_team::sync] synchronizing github
 rust_team::sync] 💻 Team Diffs:
    📝 Editing team 'rust-lang/project-assumptions-on-binders':
      Deleting member 'rust-lang-owner'
    💻 Repo Diffs:
    📝 Editing repo 'rust-lang/project-assumptions-on-binders':
      Environments:
        🔄 Update: github-pages
            Removing branches: main
    📝 Editing repo 'rust-lang/rust':
      Rulesets:
          Updating 'stable'
            Bypass Actors: [RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }] => [RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }]
          Updating 'beta'
            Bypass Actors: [RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }] => [RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }]
          Updating '*'
            Bypass Actors: [RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 16787004, actor_type: Team, bypass_mode: Always }] => [RulesetBypassActor { actor_id: 16787004, actor_type: Team, bypass_mode: Always }, RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }]
          Updating '*/**/*'
            Bypass Actors: [RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }] => [RulesetBypassActor { actor_id: 278306, actor_type: Integration, bypass_mode: Always }, RulesetBypassActor { actor_id: 217112, actor_type: Integration, bypass_mode: Always }]