Disable creating user mentions during import

Stan Hurequested to merge
sh-disable-store-mentions-import into master

Creating a note causes a subtransaction every time CacheMarkdownField attempts to refresh the issuable's user mentions. We disable it for now to avoid large number of SQL queries. We are not using this table at the moment for reads. #21801 tracks the work necessary to make this work.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/336788

Edited by Stan Hu

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