Do not add symlinks to clang-9 in the Docker.debian-testing image

Unless there's no other option we should never modify
the operating system, especially when it affects packages
unrelated to pmem. In this particular case,
if clang would point to something other than clang-9,
we could set CMAKE_C/CXX_COMPILER to the correct path.
(https://github.com/pmem/libpmemobj-cpp/pull/473#issuecomment-542313407)