net: ip: Fix compiling with 15.4

Kconfig allows selecting NETWORKING_WITH_15_4_TI_CC2520 even if the
current board doesn't support it, and also selects it by default. This
breaks building the 15.4 sample with qemu_x86. Add a config option for
having CC2520 support and enable the choise only if it is available.

In addition, remove unused function from iee802154 code, as it now
fails the tests.

Jira: ZEP-697

Change-Id: Ib082f82acdd0f86d3306bbd3bb827f61b0fd0be1
Signed-off-by: Jaakko Hannikainen <jaakko.hannikainen@intel.com>