RFR: Drop RuntimeType

[Maurizio Cimadamore]

This PR removes `RuntimeType`/`NullableClass` and related methods, as they are no longer used (after recent cleanup to `ArrayCreation`).

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Commit messages:
 - Merge branch 'bworld' into drop_runtime_type
 - Drop RuntimeType/NullableClass
 - Address review comments
 - Merge branch 'bworld' into strict_array_trans
 - Add regression test
 - Initial push

Changes: https://git.openjdk.org/valhalla/pull/1878/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1878&range=00
  Stats: 250 lines in 7 files changed: 0 ins; 248 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1878.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1878/head:pull/1878

PR: https://git.openjdk.org/valhalla/pull/1878