Remove bzero declaration

The kernel was migrated to memset in ba96f37758 ("Use __builtin
for various mem* and b* (e.g. bzero) routines.") and there are no
remaining architectures using the file.

malloc is augmented to prevent KMSAN from breaking.

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

sys/sys/malloc.h

@@ -233,7 +233,7 @@ void	*malloc(size_t size, struct malloc_type *type, int flags) __malloc_like
 		_malloc_item = malloc(_size, type, (flags) &~ M_ZERO);	\
 		if (((flags) & M_WAITOK) != 0 ||			\
 		    __predict_true(_malloc_item != NULL))		\
-			bzero(_malloc_item, _size);			\
+			memset(_malloc_item, 0, _size);			\
 	} else {							\
 		_malloc_item = malloc(_size, type, flags);		\
 	}								\

sys/sys/systm.h

@@ -342,7 +342,6 @@ void	hexdump(const void *ptr, int length, const char *hdr, int flags);
 
 #define ovbcopy(f, t, l) bcopy((f), (t), (l))
 void	bcopy(const void * _Nonnull from, void * _Nonnull to, size_t len);
-void	bzero(void * _Nonnull buf, size_t len);
 void	explicit_bzero(void * _Nonnull, size_t);
 
 void	*memset(void * _Nonnull buf, int c, size_t len);