Clean-up CSSValueKeywords.in

This change started as a removal of stale entries in the file
(mostly related to font-stretch - that was rolled out - along
with some misc values).

Then I looked at the file and found out that we still referred
to KHTML and CSS_PROP* (none of which make any sense) so I
cleaned up the comments too.

Review URL: https://codereview.chromium.org/324223004

git-svn-id: svn://svn.chromium.org/blink/trunk@175962 bbb929c8-8fbe-4397-9dbb-9b2b20218538