Split Project#reset_cache_and_import_attrs up

In EE we don't want to remove import data for mirrors. By moving this
logic to a separate method we can simply re-define the method in EE
using "prepend" and call "super" whenever necessary.

app/models/project.rb

@@ -478,7 +478,11 @@ class Project < ActiveRecord::Base
       ProjectCacheWorker.perform_async(self.id)
     end
 
-    self.import_data&.destroy
+    remove_import_data
+  end
+
+  def remove_import_data
+    import_data&.destroy
   end
 
   def import_url=(value)