closure requirements: don't replace bivariant opaque args

It is unnecessary, these get constrained when checking that the
opaque type is well-formed.

It also results in the opaque type no longer being well formed.
If you've got `fn foo<'a>() -> impl Sized + 'a` the opaque is
`type Opaque<'a, 'aDummy> where 'a: 'aDummy, 'aDummy: 'a` where
`'aDummy`  is bivariant. If we call `foo::<'b>()`  inside of a closure
and its return type ends up in a type test, we start out with the WF
`Opaque<'b, 'b>`, and then replace the bivariant `'b` with `'static`.
`Opaque<'b, 'static>`  is no longer well-formed. Given how these type
tests are used, I don't think this caused any practical issues.

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