tipc: Don't initialize link selector field in fragmented messages

Eliminates code that sets the link selector field in the header of
fragmented messages, since this information is never referenced.
(The unnecessary initialization was harmless as it was over-written
by the fragmented message identifier value before the fragments were
transmitted.)

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