wic: don't forcibly pass -T default

This reverts part of oe-core eecbe625, which was a previous attempt
to solve the Y2038 problem.  This is now solved centrally in e2fsprogs,
so doesn't need to be dealt with in wic.

We don't revert the commit entirely, to retain the warning if a
filesystem has small inodes.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>