WarpX class: make zmax_plasma_to_compute_max_step a private member variable and remove do_compute_max_step_from_zmax

[lucafedeli88]

This PR combines the following static variables of the WarpX class:

static amrex::Real zmax_plasma_to_compute_max_step;
static bool do_compute_max_step_from_zmax;

into a single private member variable:

std::optional<amrex::Real> m_zmax_plasma_to_compute_max_step = std::nullopt;

This is a small step towards reducing reliance on static class variables.

[ax3l]

Nice!