mountfs: Remove excessive sync-ing before umount

mountfs: Remove excessive sync-ing before umount

The system should perform all write operations when sync is called and
only return when the write queues are empty.

There is no additional benefit for calling sync again as the buffers
should be empty. If data is still being lost, then that is a bug in
either the storage device or driver.

As the (re-)boot process is already so slow, I would like to get rid of
any unnecessary delays.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>