Revert "test/common: add a slight stall in tearDown"

This reverts commit bb7184163e6b6fcf1c1fbc41fd0000cf59918277.

We changed test_runner.BinTahoe.run_bintahoe since this commit landed:
the new version can no longer cause the test to be skipped late (we've
gotten rid of the bin/tahoe script entirely, so it's no longer possible
for us to miss it). Hence I think we don't need this unsightly stall any
longer.

src/allmydata/test/common.py

@@ -453,8 +453,7 @@ class SystemTestMixin(pollmixin.PollMixin, testutil.StallMixin):
 
     def tearDown(self):
         log.msg("shutting down SystemTest services")
-        d = self.stall(0.001)
-        d.addCallback(lambda _: self.sparent.stopService())
+        d = self.sparent.stopService()
         d.addBoth(flush_but_dont_ignore)
         return d