[GraphQL] Investigate how to lower query count for MergeRequestSetAssignees mutator

app/graphql/mutations/merge_requests/set_assignees.rb

For some reason setting assignees on an MR using MergeRequests::UpdateService is generating excessive SQL queries

Gitlab::QueryLimiting::Transaction::ThresholdExceededError:
  Too many SQL queries were executed in GraphqlController#execute (application/json): a maximum of 100 is allowed but 138 SQL queries were executed

We need to investigate this and remove the QueryLimiting whitelist from the class