There is no HP 300 support in FreeBSD anymore, so remove the obsolete

BUGS section.

While I'm there also bump Dd date.

MFC after:	2 weeks

lib/libc/sys/reboot.2

@@ -28,7 +28,7 @@
 .\"     @(#)reboot.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd June 4, 1993
+.Dd September 18, 2015
 .Dt REBOOT 2
 .Os
 .Sh NAME
@@ -166,8 +166,3 @@ The
 .Fn reboot
 system call appeared in
 .Bx 4.0 .
-.Sh BUGS
-The HP300 implementation supports neither
-.Dv RB_DFLTROOT
-nor
-.Dv RB_KDB .