unused `must_use` warning should have an auto-fix to just remove the useless attribute

[Lokathor]

So I've got a project with 140 #[must_use] attributes on methods of trait impls. Rust tells me they have no effect when applied to a trait method:

warning: `#[must_use]` has no effect when applied to a provided trait method
    --> src/x86_x64/avx.rs:2250:3
     |
2250 |   #[must_use]
     |   ^^^^^^^^^^^

so far so good, except cargo fix can't just remove all the useless must_use for me, which it should be able to do.

[fmease]

At devs, mark that future sugg as ToolOnly/tool_only, so it doesn't show in the user's terminal which would be too verbose