[CI] clean CI, remove cmake install step

.github/workflows/ci.yml

@@ -179,10 +179,6 @@ jobs:
       - name: Install (Linux)
         if: runner.os == 'Linux'
         run: |
-          # CMake 3.15 allows specifying the generator using the CMAKE_GENERATOR
-          # environment variable.
-          curl -sSL https://github.com/Kitware/CMake/releases/download/v3.15.4/cmake-3.15.4-Linux-x86_64.tar.gz -o cmake.tar.gz
-          sudo tar xf cmake.tar.gz --strip 1 -C /usr/local
           # Required for libc6-dbg:i386 and g++-multilib packages which are 
           # needed for x86 builds.
           sudo dpkg --add-architecture i386
@@ -191,10 +187,6 @@ jobs:
           sudo add-apt-repository "deb http://dk.archive.ubuntu.com/ubuntu/ xenial main"
           sudo add-apt-repository "deb http://dk.archive.ubuntu.com/ubuntu/ xenial universe"
           
-          # LLVM 9 is not in Bionic's repositories so we add the official LLVM repository.
-          #if [ "${{ matrix.compiler }}" = "clang" ] && [ "${{ matrix.version }}" = "9" ]; then
-          #  sudo add-apt-repository "deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main"
-          #fi
           sudo apt-get update
           # libc6-dbg:i386 is required for running valgrind on x86.
           sudo apt-get install -y ninja-build valgrind libc6-dbg:i386