Remove legacy schema stuff (Cassandra 2.x and such)

[mykaul]

From Scylla's code:

         // The following 4 columns are only present up until 2.1.8 tables
         {"key_aliases", utf8_type},
         {"value_alias", utf8_type},
         {"column_aliases", utf8_type},
         {"index_interval", int32_type},},

But there are others that can be removed: key_validator, default_validator , is_dense support (compact storage, which is deprecated in Scylla) and others, all of which are in the _build_table_metadata() and other functions, making the code more spaghetti than needed.

[mykaul]

We can just compare what the Rust Driver has, which doesn't have this legacy whatsoever. We could check with the GoCQL, but it does have a bit of that legacy too.

[mykaul]

We can just compare what the Rust Driver has, which doesn't have this legacy whatsoever. We could check with the GoCQL, but it does have a bit of that legacy too.

Sent a PR to remove compact storage from GoCQL - scylladb/gocql#658