[doc](be) Remove completed new parquet plan docs

### What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary: The temporary new parquet page-level skip and observability profile planning documents described implementation work that is now completed on this branch. Keeping these design notes in docs would make the current implementation status harder to read, so this change removes the obsolete plan documents.

### Release note

None

### Check List (For Author)

- Test: No need to test (documentation cleanup only)
- Behavior changed: No
- Does this need documentation: No

Author: suxiaogang223