chore(robot-server): Delete plot_session.py

[SyntaxColoring]

Overview

robot-server has a plot_session.py script to generate state machine diagrams of the OT-2's calibration sessions. It hasn't been maintained since August 2020 and it's currently not runnable. We don't think it's worth the effort to fix, so let's delete it.

@b-cooper says:

I’m pretty sure I made this back in the day. That code is now winterized, so nuking it is the most responsible thing to do.

Also, delete an empty session.pdf file presumably left over from one of its runs.

Final PDFs for posterity

Before deleting it, I was able to get it sort of runnable. (The calibration_check session remained broken and raised TypeErrors, and the --wildcard-integrated switch remained broken and raised AttributeErrors.) The code is in this PR's commit history. Here are the PDFs that it generated:

• pipette_offset_calibration.pdf
• deck_calibration.pdf
• tip_length_calibration.pdf
• calibration_check.pdf: couldn't get this to work.

Test plan

None needed.

Review requests

Is this a good idea?

Risk assessment

No risk.