svn_wc_resolve() should remove conflict-* entry attributes

Currently, running 'svn resolve' will cleanup the conflict tmpfiles,
but leaves the conflict-* attributes still present in the entries
file.  While this is technically okay, it means that every attempt to
get the status of a resolved file must verify that those three
tmpfiles are missing from disk.  This is a missed efficiency
opportunity.  If the entry is fixed to no longer have conflict-*
attributes, these extraneous checks won't happen every time the status
of the entry is calculated.

I thought this code was behaving as I desired, sussman disagreed.  As
it turns out, the code *does exist* but is commented out.  See
resolve_conflict_on_entry() in libsvn_wc/adm_ops.c.