dgit: build_source: drop call to build_prep

dgit: build_source: drop call to build_prep

We want the callers to do this so they can control it better.  They
mostly already do.  In detail:

* cmd_push_source

  This one is most complicated.  It already calls prep_push, which
  does a variety of things which overlap with build_prep.  The things
  which are in build_prep but not in prep_push are:
   - notpushing (via build_prep_early), which seems to be a mistake
   - clean_tree(), which is needed, so we add it (for now...)
   - build_maybe_quilt_fixup, which is needed, so we add it
   - remove old changes, which is not actually needed

* cmd_build, cmd_gbp_build

  build_prep was already called by the if arm which didn't call
  build_source; so we can just move it up outside the if.  No
  functional change here.

* cmd_build_source, cmd_sbuild

  We need to replace the call.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>