Remove "token" attribute in Runners API

As we've discussed in #214320 (closed), we will remove "token" attribute from Runners API.

• backend => https://gitlab.com/gitlab-org/gitlab/-/blob/v12.9.0-ee/lib/api/entities/runner_details.rb#L13
• documentation => https://gitlab.com/gitlab-org/gitlab/-/blob/v12.9.0-ee/doc/api/runners.md#L191
• frontend => Required to check if it is used in UI.

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Suggestion from @mbobin:

Should we remove it with a feature flag before deleting the code entirely?