Revert "Fix Sitemap generation for non-file urls."

This reverts commit 8497de9413bb2982c612ea19e7d462adf03b0a78.

It fails with:

~/blog ∙ make clean html
[ ! -d /Users/jcgay/blog/output ] || rm -rf /Users/jcgay/blog/output
pelican /Users/jcgay/blog/content -o /Users/jcgay/blog/output -s /Users/jcgay/blog/pelicanconf.py
WARNING: AUTHOR_SAVE_AS is set to None
WARNING: AUTHOR_SAVE_AS is set to None
CRITICAL: 'NoneType' object has no attribute 'startswith'
make: *** [html] Error 1