vt/sc: retire logic to select vt(4) by default for UEFI boot

We previously defaulted to using sc(4) with a special case to prefer
vt(4) when booted via UEFI. As vt(4) is now always the default we can
simplify this.

Reviewed by: imp, kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45356

(cherry picked from commit 9b1de7e4844d951a7d7335cbde75a86a2380e220)