x-pack/filebeat/input/{cel,httpjson}: elide unneeded retryable HTTP client construction

[efd6]

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See title.

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

[chemamartinez]

LGTM