quota: remove legacy subcommand decoding support code

quota: remove legacy subcommand decoding support code

Support for these quota subcommands by linux kernels has been dropped
long time ago (between 2.5.16 and 2.5.17), so lets drop it in order
to minimize amount of code which should be subjected to testing and
structured conversion.

* quota.c (struct v1_dqblk, struct v2_dqblk, struct v2_dqinfo,
struct v1_dqstats, struct v2_dqstats): Remove.
(decode_cmd_data): Remove handling of Q_V1_GETQUOTA, Q_V1_SETQUOTA,
Q_V2_GETQUOTA, Q_V2_SETQUOTA, Q_V2_GETINFO, Q_V2_SETINFO, Q_V1_GETSTATS,
and Q_V2_GETSTATS subcommands.
(SYS_FUNC(quotactl)): Remove handling of Q_V1_QUOTAON subcommand.