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

This configuration was overriden ever since switching to explicit
debian/patches/ in 999a45f ,
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. 307cac9 .

Author: Beuc