staging: line6: drop midi_postprocess flag

It is no longer necessary to trigger on MIDI transmit messages that will
change the state of the device since:

1. We've dropped the midi_postprocess sysfs attr in a previous commit so
   it is not possible to activate this feature anymore.

2. The other sysfs attrs that could inspect the state after a dump
   request have been dropped.

Therefore we can safely remove this dead code.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>