MIPS: MT: Remove unused MT single-threaded cache flush code

Commit ac41f9c4 ("MIPS: Remove a temporary hack for debugging cache
flushes in SMTC configuration") removed an ugly hack that allowed cache
flushing to be performed single-threaded, something which should not be
necessary outside of debug sessions performed long ago.

Whilst the hack was removed from the cache flush code itself, the
mt_protdflush & mt_protiflush variables were left behind along with code
providing the protdflush & protiflush command line arguments. The
mt_cflush_lockdown() & mt_cflush_release() functions were also left
behind but are now entirely unused.

Remove all the unused code to complete the removal of the MT ASE
single-threaded cache flush hack.

Signed-off-by: Paul Burton <paul.burton@mips.com>