[SPARK-47972][SQL][FOLLOWUP] Restrict CAST expression for collations

### What changes were proposed in this pull request?
Removal of immutable Seq import.

### Why are the changes needed?
This import was added with #46474, but in reality is changing behaviour of other AstBuilder.scala rules and because of this needs to be removed.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Existing tests cover this.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes #46860 from mihailom-db/SPARK-47972-FOLLOWUP.

Authored-by: Mihailo Milosevic <mihailo.milosevic@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala

@@ -20,7 +20,6 @@ package org.apache.spark.sql.catalyst.parser
 import java.util.Locale
 import java.util.concurrent.TimeUnit
 
-import scala.collection.immutable.Seq
 import scala.collection.mutable.{ArrayBuffer, Set}
 import scala.jdk.CollectionConverters._
 import scala.util.{Left, Right}