Remove version field from the `Schema` struct

[evenyag]

What type of enhancement is this?

Refactor

What does the enhancement do?

Discussed in https://github.com/GreptimeTeam/greptimedb/discussions/349

^{Originally posted by evenyag October 25, 2022} Now we have RegionSchema and TableInfo that could hold the version field, so we might be able to remove the version field from the Schema struct. Since some schemas might not have version info.

Implementation challenges

We need to refactor all tests that call the SchemaBuilder::version API.