Remove repeated code

src/github/GitTree.py

@@ -2,12 +2,8 @@
 # Do not modify it manually, your work would be lost.
 
 import PaginatedList
+from GithubObject import *
 import GitTreeElement
-# This allows None as a valid value for an optional parameter
-
-class DefaultValueForOptionalParametersType:
-    pass
-DefaultValueForOptionalParameters = DefaultValueForOptionalParametersType()
 
 class GitTree( object ):
     def __init__( self, requester, attributes, lazy ):