[cleanup][ci] Remove compiler selection in CMAKE_FLAGS.

.gitlab-ci.yml

@@ -52,6 +52,6 @@ ubuntu-20.04-gcc-9-17:
   variables:
     DUNECI_TOOLCHAIN:  gcc-9-17
     DUNE_CONTROL_PATH: /duneci/modules:$CI_PROJECT_DIR
-    DUNE_CMAKE_FLAGS:   "-DDUNE_ENABLE_PYTHONBINDINGS=OFF -DCMAKE_CXX_COMPILER=clang++-10 -DCMAKE_C_COMPILER=clang-10 -DDUNE_GRID_GRIDTYPE_SELECTOR=ON -DCMAKE_CXX_FLAGS='-std=c++17 -O2 -g -Wall -fdiagnostics-color=always' -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
+    DUNE_CMAKE_FLAGS:   "-DDUNE_ENABLE_PYTHONBINDINGS=OFF -DDUNE_GRID_GRIDTYPE_SELECTOR=ON -DCMAKE_CXX_FLAGS='-std=c++17 -O2 -g -Wall -fdiagnostics-color=always' -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
     DUNECI_CMAKE_FLAGS: $DUNE_CMAKE_FLAGS
   tags: [duneci]