no uses of this remain

Index: sys/sys/buf.h
===================================================================
RCS file: /cvs/src/sys/sys/buf.h,v
diff -u -p -r1.115 buf.h
--- sys/sys/buf.h	27 Mar 2025 23:30:54 -0000	1.115
+++ sys/sys/buf.h	30 Mar 2025 09:18:05 -0000
@@ -165,9 +165,6 @@ struct bufcache {
 	struct bufqueue warmqueue;
 };
 
-/* Device driver compatibility definitions. */
-#define	b_active b_bcount		/* Driver queue head: drive active. */
-
 /*
  * These flags are kept in b_flags.
  */