staging: r8188eu: TrainIdx is set but never used

The field TrainIdx of struct fast_ant_train is set but never used.
Remove it.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20211228101120.9120-9-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>