[Improve] Remove useless ReadonlyConfig flatten feature

[Hisoka-X]

Purpose of this pull request

This PR try to remove flattern in ReadonlyConfig.
Before this PR, as default behavior, ReadonlyConfig will try to flattern config key.

// eg:
mapkey {
  inner {
   key = value
  }
}

// convert to
mapkey.inner.key = value

This feature seems unnecessary, and will affect the reading of other config that do not want flattern, and will cause errors when handling null values.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

exist test.

Check list

• If any new Jar binary package adding in your PR, please add License Notice according
  New License Guide
• If necessary, please update the documentation to describe the new feature. https://github.com/apache/seatunnel/tree/dev/docs
• If you are contributing the connector code, please check that the following files are updated:
  1. Update change log that in connector document. For more details you can refer to connector-v2
  2. Update plugin-mapping.properties and add new connector information in it
  3. Update the pom file of seatunnel-dist
• Update the release-note.

[ruanwenjun]

LGTM

[ruanwenjun]

This PR will close #5597

[hailin0]

LGTM

[ruanwenjun]

+1