[FLINK-8508][Table API & SQL] Remove RexSimplify copy from flink

[suez1224]

What is the purpose of the change

Remove RexSimplify copy from flink. This depends on https://github.com/apache/flink/pull/5791.


## Brief change log

  -  remove code.


## Verifying this change

This change is already covered by existing tests.

## Does this pull request potentially affect one of the following parts:

  - Dependencies (does it add or upgrade a dependency): (no)
  - The public API, i.e., is any changed class annotated with `@Public(Evolving)`: (y no)
  - The serializers: (no)
  - The runtime per-record code paths (performance sensitive): (no)
  - Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
  - The S3 file system connector: (no)

## Documentation

  - Does this pull request introduce a new feature? (no)
  - If yes, how is the feature documented? (not applicable)