RFR: 8364570: Remove LockingMode related code from riscv64

[Fredrik Bredberg]

Since the integration of [JDK-8359437](https://bugs.openjdk.org/browse/JDK-8359437) the `LockingMode` flag can no longer be set by the user, instead it's declared as `const int LockingMode = LM_LIGHTWEIGHT;`. This means that we can now safely remove all `LockingMode` related code from all platforms.

This PR removes `LockingMode` related code from the **riscv64** platform.

When all the `LockingMode` code has been removed from all platforms, we can go on and remove it from shared (non-platform specific) files as well. And finally remove the `LockingMode` variable itself.

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Commit messages:
 - 8364570: Remove LockingMode related code from riscv64

Changes: https://git.openjdk.org/jdk/pull/26658/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26658&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364570
  Stats: 585 lines in 9 files changed: 11 ins; 524 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/26658.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26658/head:pull/26658

PR: https://git.openjdk.org/jdk/pull/26658