Remove Swift DispatchQueue usage from tabs and places

[lougeniaC64]

Provided the work to complete issue #5012 is done and no adverse impact to the performance of logins component (or any adverse impact has been addressed), remove DispatchQueue instantiation and usage from Tabs.swift and Places.swift.

Note: DispatchQueues are also used in the rc_log and fxa-client components, though differently and more practically and purposefully than in the logins, tabs, and places components. Also it's worth noting that the database layer logic in iOS for the three latter components creates additional DispatchQueues that our API calls are wrapped in which could cause thread locking in iOS. All that to say removing DispatchQueue usage from logins, tabs, and places is the primary focus of this effort.

┆Issue is synchronized with this Jira Task ┆epic: Refactor iOS DispatchQueues