Rethink (or maybe even remove?) ``super``

Given the inheritance changes in 0.6.0 I think we should check where and how ``super`` fits into that and whether we should keep, remove or change it.
E.g. in
```
contract A { function f() virtual {} }
contract B { function f() virtual {} }
contract C is A B { function f() override(A, B) {
super.f();
} }
```
using ``super`` would defeat the purpose of the whole construction with overrides and unambiguous inheritance...

No files, tests, or entry points are named in the issue. Start by reviewing the 0.6.0 inheritance changes and the A/B/C example's use of super. Done requires deciding whether super should be kept, removed, or changed without undermining unambiguous inheritance and override behavior.

Written by the indexing model from the issue text.