linux: remove use_system_libxslt, instead use xml flag

We have weird crashes when system_libxslt != system_libxml.
Nobody has diagnosed them.  We should just use one flag for both
so people stop reporting problems with it.

BUG=42487

Review URL: http://codereview.chromium.org/1695019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@45766 0039d316-1c4b-4281-b951-d872f2087c98