Integrated: Fix signature attribute generation for null restrictions

[Maurizio Cimadamore]

The code in `Types` is already correctly handling null restrictions.
There's only a minor problem with the flag to enable null restriction not being passed recursively in nested call (e.g. for method signatures).

Since I was there, I also dropped signature generation code for type variable declarations.

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

Commit messages:
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1848/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1848&range=00
  Stats: 24 lines in 1 file changed: 0 ins; 20 del; 4 mod
  Patch: https://git.openjdk.org/valhalla/pull/1848.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1848/head:pull/1848

PR: https://git.openjdk.org/valhalla/pull/1848