Remove `LD_LIBRARY_PATH` in dependency build instructions in docs

[steindev]

Not necessary anymore due to PIConGPU standardly using RPATH.

[psychocoderHPC]

I do not find it in the documentation anymore but in many system profiles. If we can remove LD_LIBRARY_PATH from there is a question because it depends if the dependencies themselves are build with RPATH.

[chillenzer]

Is there any chance of changing this in a controlled manner? The only way, I see that this is possible would be to try this on each machine. Is that realistic? Do we still have access to all those machines?

If not, our options would be

• keep as is and close this as not planned
• do the changes where we can and remove untestable configurations
• do the changes where possible and leave as is otherwise
• do the changes everywhere risking failure of some

Any preferences?