Reduce projects and add clang for throughput slot.

Remove projects not needed for the throughput test to save time.
Add clang to compare performance between gcc and clang as soon as the lcg clang-6 works again.

I've managed to overwrite the list of projects, although probably not in a way that is ok.
Is there a nice way to overwrite the project list or am I better off just creating a new slot from scratch?

fyi @sstahl