bsdinstall: Don't install FreeBSD-base.conf

When installing e.g. 15.0-RC1, we want to get files from the 15.0-RC1
pkgbase repository; but running 'pkg upgrade' after installation should
get the latest bits build from releng/15.0.

With hat: re
MFC after: 8 hours (needed in 15.0-RC1)

(cherry picked from commit bdfc223c7a64369ed133a179c40067435a9f2cb3)