future cleanup: remove allow(clippy::use_self) once we rustup past 2021-02-26.

[eddyb]

I've been leaving #[allow(clippy::use_self)] across the codebase in places where the lint is wrong and Self can't be written (either type parameter mismatch like Foo<T> vs Foo<U>, or nested items that couldn't possibly name Self, etc.).

I suspect rust-lang/rust-clippy#6179 fixed most (if not all) of the use_self bugs, and that seems to be part of rust-lang/rust#82514, and so was likely included in the 2021-02-26 nightly.

I can't reproduce the bugs on the Rust playground, and that seems to only provide latest nightly for tools (i.e. Stable/Beta/Nightly toggle is ignored), so that seems to agree with my expectations.