Retire dialog

This is the last remaining piece of GPL software in the base system.
The installer transitioned to bsddialog four years ago, and the last
remaining dialog consumer, dpv, was turned off more than two years ago.
Retire dpv, libdpv, libfigpar (used only by dpv), and dialog itself.

Reviewed by: dteske
Differential Revision: https://reviews.freebsd.org/D55424