chore: Remove unused attribute imap_inbox_synced_at (#8822)

- imap_inbox_synced_at is no longer used; this PR removes all the references to the same.

db/migrate/20240131040316_remove_imap_inbox_syned_at_from_channel_email.rb

@@ -0,0 +1,5 @@
+class RemoveImapInboxSynedAtFromChannelEmail < ActiveRecord::Migration[7.0]
+  def change
+    remove_column :channel_email, :imap_inbox_synced_at, :datetime
+  end
+end