pySim-shell: drop backwards compat quirks for cmd2 < 2.6.2
Remove version guards for cmd2 < 2.0.0 and < 2.3.0, the Cmd2Compat and Settable2Compat wrapper classes, and the old fg/bg color API - none of these are needed since both requirements.txt and setup.py already mandate cmd2 >= 2.6.2.
