Unify HiveUnixTimestampSqlOperator and HiveToUnixTimestampSqlOperator

The two classes below both represent the unix_timestamp operator and have identical implementations.

• https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/reloperators/HiveUnixTimestampSqlOperator.java
• https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/reloperators/HiveToUnixTimestampSqlOperator.java

Probably there is a way to use one or the other and not both; having two ways of representing the same thing can bring various problems in query planning and it also leads to code duplication.