stand: Remove Spleen 32x64 fonts from INDEX

The Spleen 32x64 fonts cause some systems such as the AMD Framework 16
to boot extremely slowly. Adding screen.font="16x32" to
/boot/loader.conf is a partial workaround, but text displayed before the
boot menu still takes ~30 seconds to render on that laptop. With this
change, the same text renders immediately.

By leaving the font files in place but removing them from the INDEX,
users who want to use them can still load them manually. Once pending
work is committed to allow normal booting with these fonts, they can be
added back to the INDEX.

PR: 289007
Reviewed by: emaste, kevans, tsoome, ziaee
Fixes: 1ec2c8b2f364 (stand: Enable Spleen 32x64 font in the bootloader)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52402

(cherry picked from commit 5e82eeccd252e96370e8b73603b609622497000c)