hgweb: delete local variable instead of setting it to `None`

The previous code was flagged by PyCharm as an unused variable assignment.

Differential Revision: https://phab.mercurial-scm.org/D7754

mercurial/hgweb/hgwebdir_mod.py

@@ -486,7 +486,7 b' class hgwebdir(object):'
             )
             return res.sendresponse()
         finally:
-            tmpl = None
+            del tmpl
 
     def makeindex(self, req, res, tmpl, subdir=b""):
         self.refresh()