Deprecate `.from_inner2` from `StructuredMatrix` interface

We originally incorporated this function to support updates based `K.T @ (a @ a.T) @ K` and `K.T @ (g @ g.T) @ K`, e.g. in the private ASDL implementation.

But now we are using an update that computes `(K.T @ a) (K.T @ a).T` and `(K.T @ g) @ (K.T @ g).T` instead via `StructuredMatrix.from_inner`. Hence, we can remove `from_inner2` and the associated tests.