ci: drop RISC-V Clang test

The SiFive GitHub organization now deploys an IP allowlist which blocked
GitHub Actions, which makes this test always fail. In addition, this is
a quite different test than other non-x86 tests.

Drop it now.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>

.github/workflows/cmake.yml

@@ -480,11 +480,6 @@ jobs:
             # https://github.com/llvm/llvm-project/issues/55785
             msan-options: use_sigaltstack=0
 
-          - name: Ubuntu Clang RISC-V
-            os: ubuntu-latest
-            cmake-args: -GNinja -DCMAKE_TOOLCHAIN_FILE=./cmake/toolchain-riscv.cmake -DTOOLCHAIN_PATH=${PWD}/prebuilt-riscv-toolchain-qemu/riscv-clang -DQEMU_PATH=${PWD}/prebuilt-riscv-toolchain-qemu/riscv-qemu/bin/qemu-riscv64
-            packages: build-essential cmake ninja-build
-
           - name: Ubuntu Emscripten WASM32
             os: ubuntu-latest
             chost: wasm32
@@ -659,14 +654,6 @@ jobs:
         sudo apt-get update
         sudo apt-get install -y --allow-downgrades --no-install-recommends ${{ matrix.packages }}
 
-    - name: Download prebuilt RISC-V Clang toolchain & QEMU emulator
-      if: runner.os == 'Linux' && contains(matrix.name, 'RISC-V')
-      run: |
-        gh release download ubuntu20.04_llvm16.0.0_qemu7.0.0 --repo sifive/prepare-riscv-toolchain-qemu
-        tar zxvf prebuilt-riscv-toolchain-qemu.tar.gz
-      env:
-        GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-
     - name: Install packages (Windows)
       if: runner.os == 'Windows'
       run: |