http: refactor async_client_impl to remove a data-member

[adisuissa]

Commit Message: http: refactor async_client_impl to remove a data-member Additional Description: Minor refactor to remove the runtime_ data member, and use the (same) one already stored by the shared-config.

Risk Level: low Testing: N/A Docs Changes: N/A Release Notes: N/A Platform Specific Features: N/A

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:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/39945 was opened by adisuissa.

see: more, trace.