Remove spurious warnings when *_SAVE_AS = False

When `{AUTHOR,CATEGORY,TAG...}_SAVE_AS` is set to False in pelicanconf.py, pelican warns the user about it when generating those pages, e.g.

    WARNING: AUTHOR_SAVE_AS is set to False

This issue has already been raised in #744.

AFAICT, the various `*_SAVE_AS` settings all have a default value assigned to them that's not "False", so I've simply just removed the code that was generating these warnings.