sidekick: remove redundant explicit links

Sometimes the comments have links that point to the same thing, as in:

```
// Say something deep about [Foo][google.cloud.foo.v1.Foo]
message Foo
```

That ends up emitting code that references the same thing:

```
/// Say something deep about [Foo][google.cloud.foo.v1.Foo]
///
/// [google.cloud.foo.v1.Foo]: crate::model::Foo
struct Foo ...
```

The generated code could be simpler, just:

```
/// Say something deep about [Foo]
struct Foo ...
```

The issue names no files, tests, or entry points. Locate the Go code that generates documentation comments and resolves explicit links, then reproduce the shown example; done means redundant links are omitted while non-redundant links remain intact.

Written by the indexing model from the issue text.