Add patch file removal when no changes to chromium src for a file

[bbondy]

There's a slight problem with split patches.
If you remove all changes from a chromium source file, and you run update patches, it will not git rm the patch file. It just leaves it as is so you have to remember to manually also remove the patch file when you get the last change out from a modified chromium src file.

The way it works now is it doesn't create an empty patch file, it just leaves the patch file with the changes that were in there last.

Eventually someone will forget to manually remove the old unchanged patch file if they are doing lots of changes.