util: move "default" hidewindow to posix.py

There is a hidewindow in win32.py, which we get via windows.py

mercurial/posix.py

@@ -340,3 +340,11 b' def unlinkpath(f):'
 
 def lookupreg(key, name=None, scope=None):
     return None
+
+def hidewindow():
+    """Hide current shell window.
+
+    Used to hide the window opened when starting asynchronous
+    child process under Windows, unneeded on other systems.
+    """
+    pass

mercurial/util.py

@@ -475,14 +475,6 b' def checkwinfilename(path):'
             return _("filename ends with '%s', which is not allowed "
                      "on Windows") % t
 
-def hidewindow():
-    """Hide current shell window.
-
-    Used to hide the window opened when starting asynchronous
-    child process under Windows, unneeded on other systems.
-    """
-    pass
-
 if os.name == 'nt':
     checkosfilename = checkwinfilename
     from windows import *