tipc: Eliminate message header routines for caching destination node

Gets rid of a pair of routines that provide support for temporarily
caching the destination node for a message in the associated message
buffer's application handle, since this capability is no longer used.

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