BUF_ISLOCKED.9: drop a reference to lockstatus(9)

It's the same as lockmgr(9), which the man page already refers to.

share/man/man9/BUF_ISLOCKED.9

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
 .\" DAMAGE.
 .\"
-.Dd January 22, 2008
+.Dd September 26, 2025
 .Dt BUF_ISLOCKED 9
 .Os
 .Sh NAME
@@ -60,8 +60,7 @@ The lock is not held by anyone.
 .Xr buf 9 ,
 .Xr BUF_LOCK 9 ,
 .Xr BUF_UNLOCK 9 ,
-.Xr lockmgr 9 ,
-.Xr lockstatus 9
+.Xr lockmgr 9
 .Sh AUTHORS
 This manual page was written by
 .An Attilio Rao Aq Mt [email protected] .