Remove type checkers from operator registration because all type checking is deferred to the validation phase against the finished logical plan

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/calcite/utils/OpenSearchTypeUtil.java

@@ -77,8 +77,7 @@ public static boolean isDatetime(RelDataType fieldType) {
           || udtType == ExprCoreType.TIME;
     }
 
-    // Fallback check if type string contains EXPR_TIMESTAMP
-    return fieldType.toString().contains("EXPR_TIMESTAMP");
+    return false;
   }
 
   /**