shelve: drop unnecessary backup of dirstate for phase-based case

Regular shelve has a hack using an uncommitted transaction that's then
aborted at the end of the operation. It preserves the dirstate across
the abort, however, by saving a backup copy of it. Phase-based shelve
instead commits the transaction, so the hack shouldn't be necessary
there.

Differential Revision: https://phab.mercurial-scm.org/D5389

hgext/shelve.py

@@ -446,11 +446,8 b' def _includeunknownfiles(repo, pats, opt'
 
 def _finishshelve(repo):
     if phases.supportinternal(repo):
-        backupname = 'dirstate.shelve'
         tr = repo.currenttransaction()
-        repo.dirstate.savebackup(tr, backupname)
         tr.close()
-        repo.dirstate.restorebackup(None, backupname)
     else:
         _aborttransaction(repo)