Remove deprecation warning for --crate-type=metadata

[nrc]

UPDATE: Very easy way first bug! Mentoring instructions here.

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This didn't spend long in the wild, so we don't need to wait too long. But, Cargo does rely on the flag, so we should leave it in at least long enough to fix Cargo and for people to upgrade.

[Mark-Simulacrum]

It's been 5 months; nominating for compiler team discussion to remove --crate-type=metadata.

[nikomatsakis]

Let's remove it! All it takes, I believe, is to kill this snippet of code found here.