Remove installation of clang

This is possible thanks to
actions/runner-images#447

.github/workflows/format.yml

@@ -1,26 +1,10 @@
 name: Format sourcecode
-on:
-  push:
-    paths:
-    - '.github/workflows/format.yml'
-    - 'src/**.c'
-    - 'src/**.h'
-  pull_request:
-    types: [opened, synchronize, reopened]
-    paths:
-    - '.github/workflows/format.yml'
-    - 'src/**.c'
-    - 'src/**.h'
+on: [push, pull_request]
 jobs:
   format:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
-      - name: Install dependencies
-        run: |
-          curl https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-          sudo add-apt-repository -y 'deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-9 main'
-          sudo apt install clang-format-9
       - name: Format code
         run: |
           find src/ -type f -name '*.c' -name '*.c' | xargs clang-format-9 -i