Remove check for catalog support in libxml2

Changes have been made upstream, backported in
http://code.qt.io/cgit/qt/qtwebengine-chromium.git/commit/?h=65-based&id=eb5360b2


that make it possible to link against system libxml2 again, even if
it has been compiled with catalog support.

Thanks to Arfrever Frehtes Taifersar Arahesis for their help on this.

Change-Id: I61a49ab806f6dbbd96a2df09946c1a14062e6df5
Reviewed-by: Michal Klocek <michal.klocek@qt.io>