rc.d/sendmail: remove a obsolete upgrade seatbelt

This check was in place to aid the transition from sendmail pre-8.10.
8.10 was released in 2000. It's not possible to upgrade directly from
such as system (Freebsd 3?) to FreeBSD 15 so we can drop this.

Reviewed by: gshapiro, jhb
Differential Revision: https://reviews.freebsd.org/D49308