tipc: Create helper routine to delete unused name sequence structure

Replaces two identical chunks of code that delete an unused name
sequence structure from TIPC's name table with calls to a new routine
that performs this operation.

This change is cosmetic and doesn't impact the operation of TIPC.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>