MIPS: MT: Remove obsolete cache flush repeat code

In much the same vein as commit ac41f9c4 ("MIPS: Remove a temporary
hack for debugging cache flushes in SMTC configuration") and commit
eb75ecb1 ("MIPS: MT: Remove unused MT single-threaded cache flush
code"), remove the long obsolete ndflush & niflush command line
arguments which provided a hack that should not be useful outside of
debug sessions performed long ago.

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