py: Remove silly comment about interning keywords.

Of course, keywords are turned into lexer tokens in the lexer, so will
never need to be interned (unless you do something like x="def").

As it is now, the following on pyboard makes no new qstrs:

    import pyb
    pyb.info()

py/qstrdefs.h

@@ -2,8 +2,6 @@
 // All the qstr definitions in this file are available as constants.
 // That is, they are in ROM and you can reference them simply as MP_QSTR_xxxx.
 
-// TODO probably should add Python keywords, eg if, def, etc
-
 Q(*)
 Q(__build_class__)
 Q(__class__)