jsonfilter: drop legacy json-c support

jsonfilter: drop legacy json-c support

The cmake logic is wrong (E.G. PKG_CHECK_FOR_MODULES fails unless all
modules are found), and the legacy libjson.so name is also used by the
other libjson (for example [1]) which provides an incompatible API,
so just drop it.

Backported from libubox cbf80de7f4df61960f386cb01a899cf4228d38f3

[1] http://sourceforge.net/projects/libjson/

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>
[ wrapped commit description to 75 char limit ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>