RFR: 8291952: riscv: Remove PRAGMA_NONNULL_IGNORED

[Fei Yang]

This is similar to: https://bugs.openjdk.org/browse/JDK-8291895
Because the bug that was warned about has now been fixed, the warnings are now superfluous.
Build with GCC 11.2.0.

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Commit messages:
 - 8291952: riscv: Remove PRAGMA_NONNULL_IGNORED

Changes: https://git.openjdk.org/jdk/pull/9766/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291952
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9766.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9766/head:pull/9766

PR: https://git.openjdk.org/jdk/pull/9766