Tracking issue for Prohibit unused type parameters in impls (RFC 447)

[aturon]

rust-lang/rfcs#447

[nrc]

Nominating for backcompat-lang

[nikomatsakis]

Almost but not entirely fixed with #20593

[nikomatsakis]

Only thing left to do is to remove the "opt-out" provisions

[nikomatsakis]

There is also #22077 which is related

[aturon]

Closing, this is finished off!