RFR: 8294086: RISC-V: Cleanup InstructionMark usages in the backend

[Fei Yang]

On Wed, 21 Sep 2022 03:50:22 GMT, Xiaolin Zheng <xlinzheng at openjdk.org> wrote:

> 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.

Looks OK. Thanks.

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Marked as reviewed by fyang (Reviewer).

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