Remove conditional import for frozenset (available in Python 2.6+)

pkg_resources.py

@@ -43,10 +43,6 @@ try:
 except ImportError:
     from urllib.parse import urlparse, urlunparse
 
-try:
-    frozenset
-except NameError:
-    from sets import ImmutableSet as frozenset
 try:
     basestring
     next = lambda o: o.next()