packagegroup-core-lsb: remove GTK+

GTK+ 2 is being removed from oe-core, so remove it from the oe-core LSB
packagegroup.

Also document the fact that GTK+ 2 is no longer part of the LSB packagegroup,
and point to meta-oe for the recipe if required.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>