internal: Add delete_whitespace() to SyntaxEditor

[A4-Tacks]

• And Fix completions trait_impl desugar whitespace

Example

Before this PR

impl DesugaredAsyncTrait for () {
      fn foo(&self) -> impl Future<Output = usize> {
         $0
     }
}

After this PR

impl DesugaredAsyncTrait for () {
    fn foo(&self) -> impl Future<Output = usize> {
        $0
    }
}

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.