cql3/statements/create_index_statement: Move target validation
cql3/statements/create_index_statement: Move target validation

Move index target validation to preserve the same code structure as
Apache Cassandra and simplify support for multiple index targets.

avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla