tipc: add peer remove functionality

tipc: add peer remove functionality

This enables a user to remove an offline peer from the kernel data
structures. This could for example be useful when deliberately scaling
in peer nodes in a cloud environment.

This functionality was first merged in:
f9dec657e4 (Richard Alpe tipc: add peer remove functionality)

And later backed out (as the kernel counterpart was held up) in:
385caeb13b (Stephen Hemminger Revert "tipc: add peer remove functionality")

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>