grub.cfg: don't run search_grub usb in the main menuentry

There is literally an entire other menuentry just for this purpose.

resources/grub/config/grub.cfg

@@ -99,7 +99,6 @@ menuentry 'Load Operating System (incl. fully encrypted disks)  [o]' --hotkey='o
 #	1) raw devices and MBR/GPT partitions
 	search_grub ahci
 	search_grub ata
-    	search_grub usb
 #	2) LVM and RAID which might be used accross multiple devices
 	unset lvmvol
 	for vol in bootvol rootvol; do