[KYUUBI #7221] [DOC] Fix nonexisting document 'client/advanced/features/engine_pool'

### Why are the changes needed?

The PR fixes `nonexisting document 'client/advanced/features/engine_pool'` warning:
```shell
./kyuubi/docs/client/advanced/features/index.rst:19: WARNING: toctree contains reference to nonexisting document 'client/advanced/features/engine_pool' [toc.not_readable]
```
The document was deleted in the PR #7118.

### How was this patch tested?

Checked that there is no  warning anymoreduring the documentation build process.
```shell
make html
```

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #7221 from dnskr/doc-fix-nonexisting-document-engine_pool.

Closes #7221

806addc [dnskr] [DOC] Fix nonexisting document 'client/advanced/features/engine_pool'

Authored-by: dnskr <dnskrv88@gmail.com>
Signed-off-by: dnskr <dnskrv88@gmail.com>

Author: dnskr

docs/client/advanced/features/index.rst

@@ -22,9 +22,6 @@ Advanced Features
     engine_type
     engine_share_level
     engine_ttl
-    engine_pool
     engine_resources
     scala
     plan_only
-
-