network: remove an assert

[alyssawilk]

#16122 added asserts to utilities to make sure they were called from the correct thread.
Over on the Envoy mobile side when we do QUIC we're doing it from the main thread, and failing some of the assert checks.
I think the right thing to do here is just remove the assert

This is the only use of isWorkerThread() so I'm inclined to just remove it.
cc @goaway

Risk Level: n/a
Testing: n/a
Docs Changes: n/a
Release Notes: n/a

[goaway]

Thanks @alyssawilk! /cc @colibie

[alyssawilk]

/retest now CI is unbroken

[repokitteh-read-only]

Retrying Azure Pipelines:
Retried failed jobs in: envoy-presubmit

🐱

Caused by: a #18201 (comment) was created by @alyssawilk.

see: more, trace.

[mattklein123]

Yay red.