xtensa: remove unused XTENSA_ISS_NETWORK Kconfig parameter

This removes the XTENSA_ISS_NETWORK Kconfig parameter,
which was no longer used anywhere in the source code
and Makefiles.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>