PreTeXt: remove sidebyside

[Alex-Jordan]

This is catching up with PreTeXt developments. Formerly, PTX did not allow tabular, image or video elements without certain wrappers like figure, table, and sidebyside. We never allowed the first two as WW output, and used sidebyside as the wrapper in all cases. Now this practice is not needed. PTX allows "naked" tabular, image and video elements. Clearing away all the sidebyside stuff simplifies this WW code, and also prevents PTX from issuing warnings that it detects a pointless sidebyside.

I'd like to include this in 2.16. I suspect no one else who is active would want to test PreTeXt stuff. If you would like to review this, I suggest just eyeing the code edits to see that I didn't leave a < somewhere where it should have been cut, and typos like that.

[drgrice1]

Since these changes only change the PTX display format, I can approve this without extensive testing. I tested to make sure that problems using these macros still work. Mainly just to ensure there aren't any syntax errors introduced, as that is the only thing that would cause problems for other display formats.