remove outdated todo comments

We claim that the build dir is opaque and thus don't need to make
the object file dir overridable.

Change-Id: I42d2cf3398f00356b9e4845a9b8a8d4e27c59cd7
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>