input/wav: undo "create channels once" workaround

input/wav: undo "create channels once" workaround

Undo the essence of commit 9e850040db5c which added a workaround to
create sigrok channels only once, and keep reusing them across every
file import (without checking details). Which was obsoleted by the
recent channel list check on file re-load, including proper response
when essential file properties have changed.

Indentation changes. See a whitespace ignoring diff for the essence.