Remove a stale descriptive comment for gbincore().

The splay trees referenced in the comment were converted to
path-compressed tries in r250551.

MFC after:	3 days

sys/kern/vfs_subr.c

@@ -1612,16 +1612,7 @@ buf_vlist_add(struct buf *bp, struct bufobj *bo, b_xflags_t xflags)
 }
 
 /*
- * Lookup a buffer using the splay tree.  Note that we specifically avoid
- * shadow buffers used in background bitmap writes.
- *
- * This code isn't quite efficient as it could be because we are maintaining
- * two sorted lists and do not know which list the block resides in.
- *
- * During a "make buildworld" the desired buffer is found at one of
- * the roots more than 60% of the time.  Thus, checking both roots
- * before performing either splay eliminates unnecessary splays on the
- * first tree splayed.
+ * Look up a buffer using the buffer tries.
  */
 struct buf *
 gbincore(struct bufobj *bo, daddr_t lblkno)