Remove schema checking rake task
Remove schema checking rake task

This task will not have worked for many years as it relies on the format
field which was replaced by document_type and schema in 2016 [1].

From having a look at the Content Store there are only 871 items that
still have a format field so this is likely evidence that this has been
used in at least 6 years.

It is also not tested.

[1]: https://github.com/alphagov/content-store/commit/d135018aca4081ad367e771f00627f82927e36fe

alphagov/content-store,alphagov/content-store