Remove link to mitm.watch (site shut down)

[alesandroortiz]

Site was shut down.

Fixes issue #452.

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[alesandroortiz]

@googlebot I signed it!

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[christhompson]

Thanks for the PR! mitm.watch was a cool test so hopefully it comes back someday, but removing since it is indefinitely shut down sounds good to me.

[alesandroortiz]

No problem. Thanks for quick merge.

BadSSL has been extremely helpful and I typically use it several times a week for security analysis, so the team's and contributors' work over the years is really appreciated. Have a great week!