Remove some images from release 2.6
Remove some images in the old release. Remaining are three toolchains:
- debian-9, gcc-6, c++14
- debian-10, gcc-7, c++14 (default)
- debian-10, clang-7, libcpp, c++17
Additionally, two PDELab images are created:
- pdelab-deps, debian-10, gcc-7, c++14
- pdelab, debian-10, gcc-7, c++14