staging/lustre/ldlm: Remove unimplemented lock conversion traces.

Lock conversion is not really implemented, so let's stop
pretending here.
This removes ldlm_lock_convert, ldlm_cli_lock_convert
and ldlm_lock_downgrade.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>