tipc: remove meaningless assignment in Makefile

There is no module named tipc_diag.

The assignment to tipc_diag-y has no effect.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/Makefile

@@ -20,5 +20,3 @@ tipc-$(CONFIG_TIPC_CRYPTO)	+= crypto.o
 
 
 obj-$(CONFIG_TIPC_DIAG)	+= diag.o
-
-tipc_diag-y	:= diag.o