Fix pkg-message of sysutils/k3b

• Remove sentence that suggests that FreeBSD 7.x is still a going concern
• Remove reference to sysctl that doesn't exist in any supported version

This does not fix the problem that k3b doesn't *work*, though. Since
cdrecord no longer accepts dev=/dev/cd0 , k3b will consistently fail to
blank a CD, as reported in the linked PR.

PR: 228408
Submitted by: jwb