De-associate custom role from membership via API

Currently we can associate a user with a custom role via the API using the endpoint in ee/lib/ee/api/members.rb but there is no way to remove that association. This can probably happen in the existing PATCH endpoint but it is not possible currently because that endpoint currently calls not_found!('Member Role') unless member_role

added to epic &8408 (closed)

changed the description

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added automation:ml groupauthentication and authorization [DEPRECATED] labels

added devopsmanage sectiondev labels

mentioned in issue #370352 (closed)

added feature label

changed milestone to %15.9

set weight to 2

@jessieay - please see the following guidance and update this issue.

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	Please add typebug typefeature, typemaintenance and a subtype label to this issue.

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added typefeature label

added Deliverable label

assigned to @jessieay

added workflowin dev label

changed epic to &9360 (closed)

Moved to Epic Customer Facing MVC - Customizable Roles (&9360 - closed) since this is about the customer facing MVC, and not as much as performance

mentioned in commit c41bd81e

mentioned in merge request !110971 (merged)

mentioned in commit 75f62e7a

mentioned in commit e298ca77

mentioned in commit d226073e

added workflowin review label and removed workflowin dev label

mentioned in commit 03c146ed

added workflowverification label and removed workflowin review label

This is now on production !110971 (merged)

Closing

closed

added workflowproduction label and removed workflowverification label