Fix override_dh_auto_clean override to properly clean

The override added for #787844 is unfortunately incomplete, rendering
the clean target ineffective, and resulting into a dirty source tree.

Properly override dh_auto_clean to always return the source tree to the
pristine condition.

Remove the dh_auto_build override as well, as debhelper will
automatically run dh_auto_clean first anyway.

Closes: #1047095