The sdcard option can not only be used to write to SD cards, but also
for writing to USB sticks. Or even to write to internal storage if
pmbootstrap runs on a system booted from a live disk / seconds storage.
Drop the check. Thanks to Raymond Hackley for pointing out that this
check is not useful.
Reviewed-by: Anton Bambura <jenneron@postmarketos.org>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20231101193702.2223-1-ollieparanoid@postmarketos.org%3E