Remove i18n-inflector-rails dependency

There are no new releases anymore and the current version isn't
compatible with ruby 3.x.

As this feature wasn't really used a lot (Icelandic didn't even setup
inflections properly), it's probably not worth fighting for it, so lets
just drop it.

Related to #8369

spec/controllers/application_controller_spec.rb

@@ -124,39 +124,4 @@ describe ApplicationController, :type => :controller do
       expect(@controller.send(:after_sign_out_path_for, alice)).to eq(new_user_session_path)
     end
   end
-
-  describe "#set_grammatical_gender" do
-    it "is called on page load" do
-      expect(@controller).to receive(:set_grammatical_gender)
-      get :index
-    end
-
-    context "for inflected locales" do
-      before do
-        alice.language = :pl
-        alice.save
-      end
-
-      it "returns nil for an empty gender" do
-        alice.person.profile.gender = ""
-        alice.person.profile.save
-        get :index
-        expect(assigns[:grammatical_gender]).to be_nil
-      end
-
-      it "returns nil for an unrecognized gender" do
-        alice.person.profile.gender = "robot"
-        alice.person.profile.save
-        get :index
-        expect(assigns[:grammatical_gender]).to be_nil
-      end
-
-      it "sets the correct grammatical gender" do
-        alice.person.profile.gender = "ona"
-        alice.person.profile.save
-        get :index
-        expect(assigns[:grammatical_gender]).to eq(:f)
-      end
-    end
-  end
 end