tipc: Remove obsolete comments about routing table updates

Eliminates a block of comments that describe how routing table updates
are to be handled. These comments no longer apply following the removal
of TIPC's prototype multi-cluster support.

Note that these changes are essentially cosmetic in nature, and have
no impact on the actual operation of TIPC.

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