"Remove unnecessary async" assist should not appear in trait impls

[j-tai]

The Remove unnecessary async code assist should not be available inside trait implementations. This is because the async is required to conform to the trait interface, so the async is not "unnecessary".

Reproduce:

1. Paste the following code

   trait Trait {
       async fn foo();
   }
   
   impl Trait for () {
       ┃async fn foo() {
   
       }
   }

   ┃ denotes the cursor location.

2. Press alt+. to show code assists

rust-analyzer version: 0.3.1258-standalone

rustc version: 1.64.0 (a55dd71d5 2022-09-19)

relevant settings:

[koka831]

@rustbot claim

[weirdsmiley]

@koka831 are you still working on this?

[koka831]

hi @weirdsmiley, I just made a PR(#13508).