release: remove riscv64 and powerpc64 artifacts

Their CI workflows broke for different reasons.

I perceive these as niche platforms that aren't worth blocking
a release on. And not worth my time investigating CI problems.

.github/workflows/release.yml

@@ -98,24 +98,12 @@ jobs:
           target: armv7-unknown-linux-musleabi
           strip: arm-linux-musleabi-strip
           qemu: qemu-arm
-        - build: stable-powerpc64
-          os: ubuntu-latest
-          rust: stable
-          target: powerpc64-unknown-linux-gnu
-          strip: powerpc64-linux-gnu-strip
-          qemu: qemu-ppc64
         - build: stable-s390x
           os: ubuntu-latest
           rust: stable
           target: s390x-unknown-linux-gnu
           strip: s390x-linux-gnu-strip
           qemu: qemu-s390x
-        - build: stable-riscv64
-          os: ubuntu-latest
-          rust: stable
-          target: riscv64gc-unknown-linux-gnu
-          strip: riscv64-linux-gnu-strip
-          qemu: qemu-riscv64
         - build: macos
           os: macos-latest
           rust: nightly