scripts: check_init_priorities: drop the same priority check

Since bb590b5b6e introduced ordinals in the priority sequence, the "same
priority" case cannot happen anymore, furthermore the priority value in
the script is now the position of the function in the init sequence, so
if two devices have the same priority there's something real bad going
on.

Drop all the "same priority" handling code and tests, convert the case
into ane exception instead. Drop the init stubs as well from the test,
they are not required anymore.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>