[riscv-port-jdk17u:riscv-port] RFR: 8293050: RISC-V: Remove redundant non-null assertions about macro-assembler

[Fei Yang]

On Tue, 11 Apr 2023 09:07:22 GMT, Gui Cao <gcao at openjdk.org> wrote:

> Please review this backport to riscv-port-jdk17u.
> Backport of [JDK-8293050](https://bugs.openjdk.org/browse/JDK-8293050). 
> 
> No other changes, build success.

Marked as reviewed by fyang (Lead).

-------------

PR Review: https://git.openjdk.org/riscv-port-jdk17u/pull/40#pullrequestreview-1378984249