Remove deprecated mails_from preference

Please, use Spree::Store#mail_from_address now.

Ref solidusio#4712

core/lib/spree/app_configuration.rb

@@ -207,20 +207,6 @@ def allow_promotions_any_match_policy=(value)
     #     entity expansion attacks.
     preference :log_entry_allow_aliases, :boolean, default: true
 
-    # @!attribute [rw] mails_from
-    #   @return [String] Email address used as +From:+ field in transactional emails.
-    #   @deprecated Spree::Store#mail_from_address is used instead
-    preference :mails_from, :string, default: '[email protected]'
-    def mails_from=(value)
-      Spree::Deprecation.warn(<<~MSG) && preferences[:mail_from] = value
-        Solidus doesn't use `Spree::Config.mails_from` preference and it'll
-        removed on the next major release. Please, remove its definition in
-        `config/initializers/spree.rb`. Use the `Spree::Store#mail_from_address`
-        attribute for the default email address used as the 'From:' field in
-        transactional emails.
-      MSG
-    end
-
     # @!attribute [rw] max_level_in_taxons_menu
     #   @return [Integer] maximum nesting level in taxons menu (default: +1+)
     preference :max_level_in_taxons_menu, :integer, default: 1

core/spec/lib/spree/app_configuration_spec.rb

@@ -161,20 +161,4 @@ class DummyClass; end;
   it 'has default Event adapter' do
     expect(prefs.events.adapter).to eq Spree::Event::Adapters::ActiveSupportNotifications
   end
-
-  context 'mails_from' do
-    it 'is deprecated' do
-      expect(Spree::Deprecation).to receive(:warn).with(/Solidus doesn't use `Spree::Config.mails_from`/)
-
-      prefs.mails_from=('[email protected]')
-    end
-
-    it "still sets the value so that consumers aren't broken" do
-      Spree::Deprecation.silence do
-        prefs.mails_from=('[email protected]')
-      end
-
-      expect(prefs.mails_from).to eq('[email protected]')
-    end
-  end
 end