Revert "sleep: offer hibernation only if the kernel image still exists"

Revert "sleep: offer hibernation only if the kernel image still exists"

This reverts commit edda44605f06a41fb86b7ab8128dcf99161d2344.

The kernel explicitly supports resuming with a different kernel than the one
used before hibernation. If this is something that shouldn't be supported, the
place to change this is in the kernel. We shouldn't censor something that this
exclusively in the kernel's domain.

People might be using this to switch kernels without restaring programs, and
we'd break this functionality for them.

Also, even if resuming with a different kernel was a bad idea, we don't really
prevent that with this check, since most users have more than one kernel and
can freely pick a different one from the menu. So this only affected the corner
case where the kernel has been removed, but there is no reason to single it
out.