Fix warning on bg-gradient-variant mixin: revert #877

[deining]

With the change of #875 in place, when running ´hugo serve` (e.g. with the user guide) I now see a lot of warning messages like:

WARNING: The `bg-gradient-variant` mixin has been deprecated as of v4.5.0. It will be removed entirely in v5.
         on line 8:5 of ../assets/vendor/bootstrap/scss/mixins/_deprecate.scss, in mixin `deprecate`
         from line 21:12 of ../assets/vendor/bootstrap/scss/mixins/_background-variant.scss, in mixin `bg-gradient-variant`
         from line 37:18 of ../assets/scss/_boxes.scss, in mixin `box-variant`
         from line 127:14 of ../assets/scss/_boxes.scss
         from line 14:9 of stdin

The same issue was reported in #327 and fixed with #328 one and a half year ago. With #875, we are reverting this fix. That doesn't make sense to me.