feed: Drop Google Reader rel-via manipulation

Google Reader will be retired on 2013-07-01, so we should be able to
drop the special rel-via handling it got starting with eb04d97 (Bump
to version 2.67, 2010-09-21).

[1]: http://googlereader.blogspot.com/2013/03/powering-down-google-reader.html

Signed-off-by: W. Trevor King <wking@tremily.us>
feed: Streamline rel-via title extraction in _process_entry_content()

No functional change here, this just tightens up the Python code for
clarity.

Signed-off-by: W. Trevor King <wking@tremily.us>