Revert r318583 (libthr: use default WARNS level of 6)

Revert this while I fix RISC-V, SPARC, and probably all architectures
that use GCC.

Reported by:	kib
Pointy hat to:	vangyzen
Sponsored by:	Dell EMC

lib/libthr/Makefile

@@ -16,6 +16,7 @@ MK_SSP=	no
 
 LIB=thr
 SHLIB_MAJOR= 3
+WARNS?=	3
 NO_WTHREAD_SAFETY=1
 CFLAGS+=-DPTHREAD_KERNEL
 CFLAGS+=-I${SRCTOP}/lib/libc/include -I${.CURDIR}/thread \