qt5: remove qt5jsbackend package

Upstream did not release a new version of this package since it is
superseded by the builtin qml engine found in new qt5declarative.

Fixes: http://autobuild.buildroot.net/results/3861aa91e03154c7d20f4157f0c3a1fa14838af1/



Signed-off-by: Fatih Aşıcı <fatih.asici@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>