[6.x] Remove side-by-side field setting

[jasonvarga]

This PR removes the side-by-side setting which must've been added at some point to support the config style forms. But that was officially added in a different way. We don't want to allow you to set side-by-side on specific fields within any publish form as it introduces a whole array of potential headaches (styling fieldtypes at different widths, etc).

This also does some related cleanup.

[AtmoFX]

@jasonvarga Is it too late to challenge this change?
I had created #13144 to provide a possible solution to the side-by-side fields and honestly, I think it solved that "whole array of potential headaches" you are talking about in a very simple manner.

Just created https://play.tailwindcss.com/2xR9miJLCw for you to see that very little changes to make all configurations I could think of work (i.e. all field widths + all placements for the instructions).

Duncan has mentioned in #13101 that I would find the reasoning about why this is being removed but if I am being honest, I do not find a lot of reasoning here 😕

[jasonvarga]

Maybe we can bring it back one day but for now it's not something we want to maintain or introduce in a derpy state.

The headaches I was talking about wasn't necessarily just the label/instructions placements, but the fieldtypes themselves. e.g. stick an entries or assets fieldtype in there instead of text.

Thanks for understanding!