x-pack/filebeat/docs/inputs/cel: remove reference to ext package

[efd6]

This extension set is intentionally not included.

What does this PR do?

It removes an incorrectly included reference to the CEL ext extension package which is not part of the built system.

Why is it important?

The documentation currently will confuse users.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Updates Filebeat CEL input - string library functions not recognized #34610

Use cases

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Logs

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @efd6? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

💚 Build Succeeded

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• Start Time: 2023-02-20T23:44:26.196+0000

• Duration: 9 min 58 sec

❕ Flaky test report

No test was executed to be analysed.

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[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[ShourieG]

LGTM