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

[Gui Cao]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8293050](https://bugs.openjdk.org/browse/JDK-8293050). 

No other changes, build success.

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Commit messages:
 - backport RISC-V: Remove redundant non-null assertions about macro-assembler

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/40/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=40&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293050
  Stats: 53 lines in 8 files changed: 0 ins; 47 del; 6 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/40.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/40/head:pull/40

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/40