Remove CSS declarations which change default scrollbar width

[luixxiul]

Fixes SchildiChat/schildichat-desktop#238

Setting the declaration "scrollbar-width: thin" has a legitimate accessibility concern, though doing so could have made sense previously for some reason. Let's prefer each OS and its desktop environments' setting over Element's aesthetic view.

See: https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-width#accessibility

Before:

before.mp4

After:

after.mp4

Closes SchildiChat/matrix-react-sdk#24

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• I agree to release my changes under this project's license

[SpiritCroc]

Thanks!

For keeping merge conflicts flow, I'd prefer

• Commenting out code instead of fully removing it (ideally add something like a "SC: disabled" comment at the top so we know it was us when looking at the code)
• Not changing upstream comments if the benefit is negligible
• Not changing copyright headers (but it's ok if you insist to have your name there)

[luixxiul]

The file has not been updated since more than four years so it will not be changed for a while IMHO, but I'll address them anyway.

[su-ex]

Watch out doing this. I wanted to do do this a while back, but this made some mess in Firefox or some other browser. So test with every browser first.

[luixxiul]

I indeed have tested with Firefox, and am not quite sure how removing a declaration which is not recommended breaks UI, but I marked this as draft.

[SpiritCroc]

The file has not been updated since more than four years so it will not be changed for a while IMHO, but I'll address them anyway.

I don't have enough merging history to tell for desktop but on Android I was proven wrong often enough with this kind of assumption. So I think it's good to be consistent with aiming for the lowest merge conflict impact possible

[SpiritCroc]

I would put it right above the actual commented out section, and use the same /*. The first point further reduces merge conflict area, the second one is just matter of taste I guess