50_prompt: Don't export PS{1...4} and RPS{1|2} any more

50_prompt: Don't export PS{1...4} and RPS{1|2} any more

Exporting the prompt environment variables fails with non-ZSH sub-shells,
and with AX-ZSH themes that relay on environment variables that aren't
exported (and plain ZSH doesn't export by default, so nobody can relay on
this anyway).

Now themes are initialized in the "zshrc" step (insetad of "zlogin").