Docs: drop unused .bi-exclamation-triangle-fill class

[julien-deramond]

Description

While checking the several docs ongoing PRs, I stumbled upon this .bi-exclamation-triangle-fill class that is useless.
Feel free to close the PR if it's already handled in another PR.

Note: I've checked the presence of other .bi-* classes and it was the only one

[XhmikosR]

I think we should actually keep the classes and IDs in the icons so that we are close to the upstream code for easier copy-paste. We just need to be consistent from now on and update the sprite and everything from icons to be as close as the upstream code.

[mdo]

I leave this to the two of you :). Doesn't matter to me.