Remove "negotiate" from support auth scheme on iOS.
Remove "negotiate" from support auth scheme on iOS.

The support for "negotiate" is disabled on iOS when building //net,
so remove the scheme from the list of supported scheme (lying is a
terrible sin).

BUG=556566

Review URL: https://codereview.chromium.org/1489513002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#362142}

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