[riscv-port] RFR: 8282466: riscv: Remove unused code in linux_riscv

[Feilong Jiang]

This PR has the following changes:

- Remove unused code in linux_riscv
- More renaming from riscv64 to riscv in riscv backend

Hotspot and jdk tier1 tests on QEMU are passed without new failures.

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Commit messages:
 - 8282466: riscv: Remove unused code in linux_riscv

Changes: https://git.openjdk.java.net/riscv-port/pull/62/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=62&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282466
  Stats: 124 lines in 23 files changed: 0 ins; 6 del; 118 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/62.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/62/head:pull/62

PR: https://git.openjdk.java.net/riscv-port/pull/62