removed unused step_index arg from restart

[shimwell]

While looking through the openmc.deplete.abc file I noticed _get_bos_data_from_restart takes an step_index argument that is not used in the method. Perhaps we can remove this argument. I grepped for use of this method and it was only used in a couple of places so I updated those to no longer pass in the argument.

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)

[paulromano]

Thanks!