Remove `aprun to jsrun` section of Summit User Guide

[jack-morrison]

This section, "aprun to jsrun," is around from when both Titan and Summit were in production. Now that Titan has been decommissioned for several months, and Summit in production for almost 1 year, this issue proposes we remove this comparison and relocate remaining relevant information into other sections as appropriate. This should help (albeit only a small amount) to reduce the already pretty crowded jsrun section.