net: purge NET_STACK and other stack APIs

The current design of the network-specific stack dumping APIs
is fundamentally unsafe. You cannot properly dump stack data
without information which is only available in the thread object.

In addition, this infrastructure is unnecessary. There is already
a core shell command which dumps stack information for all
active threads.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>