Use PROGS instead of PROG and remove unnecessary SRCS?= assignment

Using PROG instead of PROGS will in cases of high -j with -DNO_ROOT cause
the PROG to show up more than once as it's handling the SCRIPTS install case
in a recursive manner, separate from the non-recursive case

After the recent batch of commits to bsd.progs.mk to fix behavior with how
variables are defaulted to, explicitly setting SRCS for a PROG is no longer
required

MFC after: 1 week
Reviewed by: asomers
Phabric: D1130
Sponsored by: EMC / Isilon Storage Division

tests/sys/netinet/Makefile

@@ -3,9 +3,8 @@
 TESTSDIR=	${TESTSBASE}/sys/netinet
 BINDIR=		${TESTSDIR}
 
-ATF_TESTS_SH+=	fibs_test
-PROG=	udp_dontroute
-SRCS=	udp_dontroute.c
+ATF_TESTS_SH=	fibs_test
+PROGS=	udp_dontroute
 MAN=
 WARNS?=	6