Don't store available data on disputes (#5950)

* Don't store available data on disputes

If there are lots of disputes, this leads to blowing up disk space on
validators. Rob luckily remembered that we do store the full
availability in participation.

The argument in the code does not make too much sense with the current
implementation, as no validator will ever request anything else from us,
than the one piece we are meant to posess.

* Fix warnings.

* Fix compile warnings

* Remove redundant field.

Co-authored-by: Vsevolod Stakhov <vsevolod.stakhov@parity.io>