RFR: 8374545: Drop logic for unchecked nullness conversions

[Maurizio Cimadamore]

This PR removes the recursive checks for nullness conversions.
We might need more warnings in the future, but of a much simpler kind, so the best way forward is to restore the code in `Types` and `Check` closer to what's in mainline, and take it from there.

I've left two changes:
* the sutyping test from `null` to `Foo!` still fails
* a cast from `null` to `Foo!` still fails

We might want to detune these as warnings (or not).

-------------

Commit messages:
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1857/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1857&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374545
  Stats: 250 lines in 7 files changed: 3 ins; 206 del; 41 mod
  Patch: https://git.openjdk.org/valhalla/pull/1857.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1857/head:pull/1857

PR: https://git.openjdk.org/valhalla/pull/1857