Remove flag for user_mapping_service_account_and_bots

Thiago Figueirórequested to merge
573124-ff-user_mapping_service_account_and_bots-support-reassignment-to-more-user-types into master

What does this MR do and why?

Remove flag for user_mapping_service_account_and_bots

The flag has been enabled by default since %18.5 with no ill symptoms.

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Related to #573124 (closed)

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