pluto: remove unnecessary auth header in hyper-proxy

[etungsten]

Issue number:
N/A

Description of changes:
We should only need the proxy auth header to talk to the proxy when proxying.

Testing done:
Launched instance with https_proxy setting:

[settings.network]
https-proxy = "http://user:pass@proxy:port"

Cloudtrail shows the describe instance originating from pluto going through my proxy:

    "eventTime": "2023-12-08T04:09:29Z",
    "eventSource": "ec2.amazonaws.com",
    "eventName": "DescribeInstances",
    "awsRegion": "us-west-2",
    "sourceIPAddress": "my_proxy",
    "userAgent": "aws-sdk-rust/0.55.3 os/linux lang/rust/1.74.0",

Without the https_proxy setting, the source of the event is my instance public IP as expected:

    "eventSource": "ec2.amazonaws.com",
    "eventName": "DescribeInstances",
    "awsRegion": "us-west-2",
    "sourceIPAddress": "my_instance_pub_ip",
    "userAgent": "aws-sdk-rust/0.55.3 os/linux lang/rust/1.74.0",

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[jpculp]

LGTM pending testing.