[utils] Remove Python 2

The library `six` provides compatibility between Python 2, and 3. It's no
longer necessary once we migrate of Python 2 completely.

Also remove any custom logic for Python 2 (the ones referenced by
a commentanyways).

https://bugs.swift.org/browse/SR-16025

utils/GYBUnicodeDataUtils.py

@@ -355,9 +355,6 @@ class UnicodeTrieGenerator(object):
                 else:
                     return idx
 
-            # NOTE: Python 2's `map` function returns a list. Where Python 3's
-            # `map` function returns an iterator. To work around this the
-            # result of the `map` is explicitly converted to a `list`.
             return list(map(map_index, indexes))
 
         # If self.bmp_data contains identical data blocks, keep the first one,