perf: drop an unused variable assignment

Caught by PyCharm.  A different formatter is created below in `_displaystats()`.

Differential Revision: https://phab.mercurial-scm.org/D7744

contrib/perf.py

@@ -2220,9 +2220,6 b' def perfhelperpathcopies(ui, repo, revs='
 
     fm.end()
     if dostats:
-        # use a second formatter because the data are quite different, not sure
-        # how it flies with the templater.
-        fm = ui.formatter(b'perf', opts)
         entries = [
             ('nbrevs', 'number of revision covered'),
             ('nbmissingfiles', 'number of missing files at head'),