Revert "Call the HttpClient constructor when connecting a WebSocket to allow HttpOverrides to work."

This reverts commit 92e7989fbcf0296231df34925f4333d124b30138.

Reason for revert: Causes VmService WebSocket creating to fail because HttpClient is mocked during Flutter Widget tests. See https://github.com/flutter/flutter/issues/138413

Original change's description:
> Call the HttpClient constructor when connecting a WebSocket to allow HttpOverrides to work.
>
> One consequence of this is that there will be one HttpClient per WebSocket connection rather than one for all WebSocket connections. This does *not* affect connection pooling because the sockets are detached from the HttpClient after the WebSocket connection is made.
>
> Closes https://github.com/dart-lang/sdk/pull/52509
>
> GitOrigin-RevId: 28ca642a56e3bbc6112df844b07026b9e0ae8ae6
> Change-Id: I8d9c6f2a883b0f79bafca30abd1dfd98291cf0e0
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/305620
> Reviewed-by: Alexander Aprelev <aam@google.com>
> Commit-Queue: Brian Quinlan <bquinlan@google.com>

Change-Id: Ie1d029e7acf477503cd601f6a03e121cd17d1695
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/336100
Auto-Submit: Brian Quinlan <bquinlan@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>