Remove unneeded #[allow] attributes

[bengsparks]

Motivation

Cleanup from work done in #2904.

Solution

Remove all warning suppressions related to unreachable patterns, now that an exception for empty match arms has been made and backported to beta

$ c +stable b 
   Compiling axum v0.8.0-alpha.1 (/private/tmp/axum/axum)
   Compiling axum-extra v0.10.0-alpha.1 (/private/tmp/axum/axum-extra)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.92s

$ c +beta b  
   Compiling axum v0.8.0-alpha.1 (/private/tmp/axum/axum)
   Compiling axum-extra v0.10.0-alpha.1 (/private/tmp/axum/axum-extra)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.28s

$ c +nightly b
   Compiling axum v0.8.0-alpha.1 (/private/tmp/axum/axum)
   Compiling axum-extra v0.10.0-alpha.1 (/private/tmp/axum/axum-extra)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.30s

[mladedav]

Thanks for following up on this!

[jplatte]

I was a bit confused when seeing the PR title - I don't see the builtin #[allow] attribute as a macro (no idea whether it is implemented as one). I'll update the PR title just so it's a bit more obvious (even though it's already merged).