[IMAP Resource] Remove RemoveItemsTask and use ChangeItemsFlagsTask instead

Removing emails means appending the \Deleted flag, so RemoteItemsTask is
just a simpler version of ChangeItemsFlagsTask with hardocded flag to append.
To avoid code duplication and make use of flags-related fixes in ChangeItemsFlagsTask,
we can just use that instead of RemoveItemsTask.

REVIEW: 120447
CCBUG: 339565