Assume Glitch::KeywordMute#destroy! works and error out if it doesn't.

There's nothing useful we can display if the destroy action messes up,
so might as well assert it does and complain loudly if it doesn't.

app/controllers/settings/keyword_mutes_controller.rb

@@ -33,12 +33,9 @@ class Settings::KeywordMutesController < ApplicationController
   end
 
   def destroy
-    if @keyword_mute.destroy
+    @keyword_mute.destroy!
+
     redirect_to settings_keyword_mutes_path, notice: I18n.t('generic.changes_saved_msg')
-    else
-      # FIXME
-      redirect_to settings_keyword_mutes_path, notice: "huh that didn't work right"
-    end
   end
 
   def destroy_all