packagegroup-qte-toolchain-target: remove bbappend content

This added Qwt to the Qt Embedded toolchain. This is a distro policy
decision, and in any case Qwt is a third-party library which is not part
of Qt. Distros that wish to do this should add this bbappend to their
own layers.

(We have to preserve the PRINC value to avoid PR going backwards.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>