remove flang15 from checks (not functional on trixie)

.gitlab-ci.yml

@@ -55,8 +55,6 @@ compatibility_stage:
       - make clean && BUILDDIR=$PWD/../build_gnu13 CXX=g++-13 FC=gfortran-13 make -j
       - echo "Running make with gnu compilers version 14..."
       - make clean && BUILDDIR=$PWD/../build_gnu14 CXX=g++-14 FC=gfortran-14 make -j
-      - echo "Running make with flang version 15 and clang version 15..."
-      - make clean && BUILDDIR=$PWD/../build_clang15 CXX="clang++-15 -stdlib=libstdc++ -I/usr/include/c++/13 -I/usr/include/x86_64-linux-gnu/c++/13" FC=flang-new-15 FCFLAGS=-O3 LDFLAGS="-L/usr/lib/llvm-15/lib -L/usr/lib/gcc/x86_64-linux-gnu/13" make -j
       - echo "Running make with flang version 16 and clang version 16..."
       - make clean && BUILDDIR=$PWD/../build_clang16 CXX="clang++-16 -stdlib=libstdc++ -I/usr/include/c++/13 -I/usr/include/x86_64-linux-gnu/c++/13" FC=flang-new-16 FCFLAGS=-O3 LDFLAGS="-L/usr/lib/llvm-16/lib -L/usr/lib/gcc/x86_64-linux-gnu/13" make -j
       - echo "Running make with flang version 17 and clang version 17..."