[5.6] Fix withCount() and withoutGlobalScopes()

[staudenmeir]

withoutGlobalScopes() doesn't work on withCount() relationships:

class Post extends Model {
    use SoftDeletes;
}

class User extends Model {
    public function allPosts() {
        return $this->hasMany(Post::class)->withoutGlobalScopes();
    }
}

User::withCount('allPosts')->get();

# expected
select "users".*,
  (select count(*)
   from "posts"
   where "users"."id" = "posts"."user_id")
  as "all_posts_count"
from "users"

# actual
select "users".*,
  (select count(*)
   from "posts"
   where "users"."id" = "posts"."user_id"
     and "posts"."deleted_at" is null) <--- The SoftDeletes scope should be missing.
  as "all_posts_count"
from "users"

This is caused by the way withCount() and mergeConstraintsFrom() merge the scopes.

We can fix it by explicitly adding all the scopes to $removedScopes (via withoutGlobalScope()).