Drop unused debian/source/local-options:single-debian-patch

This configuration was overriden ever since switching to explicit debian/patches/* in 999a45ff , switching away from the single debian/patches/debian-changes patch, previously implicitely generated by diffing with the upstream branch.

This is now a bit confusing so it's best to drop it for clarity.

Note: local-options is not exported in source releases (.dsc), so it also got removed in debian/xxx branches whenever a contributor worked from the archive .dsc (rather than Git) and imported the result later with gbp import-dsc, e.g. 307cac90 .