net: net_context: Drop callbacks in net_context_put()

The context may live for a while after the user closes it (c.f. TCP),
and of course the documentation specifies that the user must not use
it after calling net_context_put().  Don't confuse them by invoking
their callbacks on the "closed" connection; it's likely that the user
has destroyed her own tracking data and the user_data pointers would
be garbage.

Change-Id: Iba9cc7025c6ea4a94cc4796903966f8d1b831996
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>