Delete feed.css

It was a stylesheet loaded with our RSS and Atom feeds, intended to make
them legible when shown in a regular browser. Browsers these days tend
to have their own UI for feeds (for example IE 11 and Firefox 31), and
these that don't pretty-print the XML, which means the stylesheet
provides very little added value.

Per discussion on I6268d663.

Bug: 69277
Change-Id: I2ce9650c01f8f245707f5e88ee46520c5e598ce4
Merge "Delete feed.css"