Remove redundant explicit links

[coryan]

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 ...