Remove `RawSpec` from `ChangesetSpec` and drop column

Created by: mrnugget

We don't ever use that field and it takes up a lot of space, essentially doubling the size of the changeset_specs table.

We added it years ago because we didn't know whether we need it or not, but now we're pretty sure we don't need it.

https://github.com/sourcegraph/sourcegraph/blob/8195915c1330a77628ebb5bf40343a07bd15e0b0/enterprise/internal/batches/types/changeset_spec.go#L30

1. Remove the field
2. Drop the column