boot: remove random-seed-mode

Now that the random seed is used on virtualized systems, there's no
point in having a random-seed-mode toggle switch. Let's just always
require it now, with the existing logic already being there to allow not
having it if EFI itself has an RNG. In other words, the logic for this
can now be automatic.

src/shared/bootspec.h

@@ -57,7 +57,6 @@ typedef struct BootConfig {
         char *auto_entries;
         char *auto_firmware;
         char *console_mode;
-        char *random_seed_mode;
         char *beep;
 
         char *entry_oneshot;