Remove depreciated solvers code and tests from old interface

Revert "Skip broken tests correctly"

This reverts commit c675c6f64c9a13b46dcb36d292af12482ffff5c6.

Delete depreciated code

To be reintroduced in ITensorNetworksNext

Squash of ^

This removes `solvers/previous_interfaces` and the broken test files

Author: Jack Dunham