zetacore : delete historical migration scripts

[kingpinXD]

We usually keep previous versions of migration scripts on the node repo, but there is no fixed convention for how long we should keep them.
For example in crosschain module we have v2,v3,v4 and v5 , for observer we have only v8,v9,v10

We don’t need to keep any scripts once the migration is done.

We can remove all unnecessary migration scripts.
I would still like to keep just the latest one that ran last release and any upcoming migrations

[gartnera]

I think it is useful for developers to be able to easily reference old migrations when writing their new migrations. If they are a maintenance burden, I think deleting some (but not all) is ok.