formatter: remove now-unnecessary check for file-ness

`templater.templatepath()` now returns non-`None` only for files, so
the caller doesn't have to check.

Differential Revision: https://phab.mercurial-scm.org/D8805

mercurial/formatter.py

@@ -584,7 +584,7 b' def lookuptemplate(ui, topic, tmpl):'
         mapname = templater.templatepath(
             b'map-cmdline.' + tmpl
         ) or templater.templatepath(tmpl)
-        if mapname and os.path.isfile(mapname):
+        if mapname:
             return templatespec(topic, None, mapname)
 
     # perhaps it's a reference to [templates]