It was only needed for upgrades from GRUB 1.99 (now a long time ago) and
can inappropriately hide problems when /etc/grub.d/00_header should have
been updated but wasn't.
Closes: #953201
QUIET_BOOT is always defined: it is 1 if --enable-quiet-boot is passed
to configure, and 0 otherwise. But every CPP conditional was based on
whether it is defined or not; so the --enable-quiet-boot code paths were
in fact always enabled.