tipc: Eliminate useless check when creating internal message

Gets rid of code that allows tipc_msg_init() to create a short
payload message header. This optimization is possible because
there are no longer any callers who require this capability.

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