dh: Remove obsolete optimisation hack that caused sequence breakage in v9 with a...

dh: Remove obsolete optimisation hack that caused sequence breakage in v9 with a rules file with an explict build target. Closes: #634784

This hack was necessary back when dh ran each target, and so recursively
invoked itself. If debian/rules binary ran debian/rules binary-arch ran
debian/rules install-arch ran debian/rules build-arch, then debhelper
commands would be running with -a throughout, and so for debian/rules
binary-indep it would have to re-run all the commands with -i. The hack
avoided this extra work (and expecially dh_auto_configure running twice) by
first running the common commands without -i or -a and only then following
through with running the explicit per-arch targets, which didn't run many
(if any) additional commands.

But now dh does not run implicit targets, so (unless targets
are explicit), it will instead just construct a sequence of debhelper
commands to run directly, and so the -a flag is avoided.