RFR: JVM detect null checked flag

[Chen Liang]

`test/jdk/valhalla/valuetypes/Reflection.java` confirms that a value `!` field now generates the same VM field flag as `@NullRestricted`; this is verified by `ValueClass.isNullRestrictedField` which reads from the VM field flag.

We can now throw away the javac hack that injects `@NullRestricted`.

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Commit messages:
 - JVM detect null checked flag

Changes: https://git.openjdk.org/valhalla/pull/2041/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2041&range=00
  Stats: 94 lines in 16 files changed: 20 ins; 48 del; 26 mod
  Patch: https://git.openjdk.org/valhalla/pull/2041.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2041/head:pull/2041

PR: https://git.openjdk.org/valhalla/pull/2041