Remove all IE<11 mentions and compatibility

[vever001]

We restored the rake convert approach in bootstrap-sass, see entreprise7pro/bootstrap-sass#1

But noticed a lot of changes in SASS were made manually there, and now lost, see entreprise7pro/bootstrap-sass#2 (comment).
Those changes were mostly to remove IE mentions and compatibility, since jquery 4 drop support for IE<11.
All of these need to be done in this repo instead (entreprise7pro/bootstrap), in less/JS files, since bootstrap-sass only fetches JS/LESS from this repo (i.e. no manual changes to JS or SASS files should happen in entreprise7pro/bootstrap-sass).

This issue is to restore all those changes, or those that are still relevant.
And generally, remove all IE<11 mentions and compatibility.

[joejoseph00]

Thanks for creating this issue, yes let's move forward with this! Thanks ya it was an oversight on my part since as I ignored the .less completely.