[XFS] Remove bogus assert

This assert is bogus. We can have a forced shutdown occur
between the check for the XLOG_FORCED_SHUTDOWN and the ASSERT. Also, the
logging system shouldn't care about the state of XFS_FORCED_SHUTDOWN, it
should only check XLOG_FORCED_SHUTDOWN. The logging system has it's own
forced shutdown flag so, for the case of a forced shutdown that's not due
to a logging error, we can flush the log.

SGI-PV: 972985
SGI-Modid: xfs-linux-melb:xfs-kern:30029a

Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>