rcutil: don't check if defaultrc/ is a directory -- we know it is

`mercurial/defaultrc/` is a directory both in the Mercurial repo and
once installed on a target platform. The directory was created in
c4ce077588d0 (config: introduce "built-in" default configuration
settings in default.d, 2014-09-04). That commit has some more
information, but it still doesn't seem to say that `defaultrc/` (then
called `default.d/`) could be a file. Perhaps the check was there to
allow you to run the same code on an older install/repo?

Differential Revision: https://phab.mercurial-scm.org/D7624

mercurial/rcutil.py

@@ -63,11 +63,8 b' def envrcitems(env=None):'
 
 def defaultrcpath():
     '''return rc paths in defaultrc'''
-    path = []
     defaultpath = os.path.join(resourceutil.datapath, b'defaultrc')
-    if os.path.isdir(defaultpath):
-        path = _expandrcpath(defaultpath)
-    return path
+    return _expandrcpath(defaultpath)
 
 
 def rccomponents():