sdcard_sd: remove unused .reset() method

sdcard_sd: remove unused .reset() method

The decoder's .reset() method seems to partially implement constructor's
assignments, but is not referenced anywhere. There is neither a direct
call site in the remainder of pd.py which uses the "reset" name, and
runtime computation only references "handle_*()" methods for commands
and responses.

Drop the unused .reset() method in the individual decoder before the
introduction of a common .reset() method approach for all decoders.