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

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2024_01_29_080827) do
+ActiveRecord::Schema[7.0].define(version: 2024_01_31_040316) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
   enable_extension "pg_trgm"
@@ -276,7 +276,6 @@ ActiveRecord::Schema[7.0].define(version: 2024_01_29_080827) do
     t.string "imap_login", default: ""
     t.string "imap_password", default: ""
     t.boolean "imap_enable_ssl", default: true
-    t.datetime "imap_inbox_synced_at", precision: nil
     t.boolean "smtp_enabled", default: false
     t.string "smtp_address", default: ""
     t.integer "smtp_port", default: 0