tipc: Remove user registry subsystem

Eliminates routines, data structures, and files that make up TIPC's
user registry. The user registry is no longer needed since the native
API routines that utilized it no longer exist and there are no longer
any internal TIPC services that use it.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>