Legacy planner incompatible with Timestamp backed by LocalDateTime

Recently I upgraded a simple application that inserts static data into a table from 1.9.0 to 1.10.0, and
encountered a timestamp type incompatibility problem during the table sink validation.

The SQL is like:
```
insert into kafka.test.tbl_a # schema: (user_name STRING, user_id INT, login_time TIMESTAMP)
select ("ann", 1000, TIMESTAMP "2019-12-30 00:00:00")
```

And the error thrown:
```
Field types of query result and registered TableSink `kafka`.`test`.`tbl_a` do not match.
Query result schema: [EXPR$0: String, EXPR$1: Integer, EXPR$2: Timestamp]
TableSink schema: [user_name: String, user_id: Integer, login_time: LocalDateTime]
```

After some digging, I found the root cause might be that since FLINK-14645 timestamp fields defined via TableFactory had been bridged to LocalDateTime, but timestamp functions are still backed by java.sql.Timestamp.