Fix GRUB bootloader config dir: grub-pc → grub-efi

Build uses --bootloaders "grub-efi,syslinux" so live-build reads
config/bootloaders/grub-efi/ for the UEFI GRUB config. The directory
was incorrectly named grub-pc, causing live-build to ignore our custom
grub.cfg and generate a default one (missing toram, GSP-off entries).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/config/bootloaders/grub-pc/theme.cfg

@@ -1,9 +0,0 @@
-set color_normal=light-gray/black
-set color_highlight=yellow/black
-
-if [ -e /boot/grub/splash.png ]; then
-    set theme=/boot/grub/live-theme/theme.txt
-else
-    set menu_color_normal=yellow/black
-    set menu_color_highlight=white/brown
-fi