Disable multiprocessing on Windows python2.7

Add protection against Pool initialization failing on us (platform not supported, but import passes)

Author: bmartinn

src/pip/__main__.py

@@ -23,11 +23,4 @@
 from pip._internal.cli.main import main as _main  # isort:skip # noqa
 
 if __name__ == '__main__':
-    try:
-        # windows multiprocessing support
-        from multiprocessing import freeze_support
-        freeze_support()
-    except ImportError:
-        pass
-
     sys.exit(_main())