package/qjson: remove qt4 support

Qt4 support is about to be dropped, so remove the support from qjson as
well.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Arnout: also remove redundant BR2_STATIC_LIBS dependency/comment]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>