Fixes #26953 (closed).
We use more $input- variables than $input-btn-, but we had a few loose ends that made it a bit inconsistent in how we re-used those. In general now, we have $input-btn- variables as an abstraction layer to connect our inputs and buttons together. When the styles are for inputs only, they should be using the $input- variables instead of $input-btn- ones.
/cc @twbs/css-review