RFR: 8294086: RISC-V: Cleanup InstructionMarks in the backend

[Xiaolin Zheng]

This is a trivial cleanup of dummy `InstructionMark` usages in the RISC-V backend. There are no more `InstructionMark` usages in the backend now.

Tested hotspot tier1 and tier2 on Qemu.

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Commit messages:
 - RISC-V: Cleanup InstructionMarks in the backend

Changes: https://git.openjdk.org/jdk/pull/10369/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10369&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294086
  Stats: 21 lines in 3 files changed: 0 ins; 12 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/10369.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10369/head:pull/10369

PR: https://git.openjdk.org/jdk/pull/10369