riscv: retire bzero

Unused since ba96f37758 ("Use __builtin for various mem* and b* (e.g. bzero)
routines.")

Reviewed by:	mhorne
Sponsored by:	Rubicon Communications, LLC ("Netgate")

sys/riscv/riscv/machdep.c

@@ -979,14 +979,3 @@ initriscv(struct riscv_bootparams *rvbp)
 
 	TSEXIT();
 }
-
-#undef bzero
-void
-bzero(void *buf, size_t len)
-{
-	uint8_t *p;
-
-	p = buf;
-	while(len-- > 0)
-		*p++ = 0;
-}