Revert "Add align-self: center to buttons for improved rendering in flex containers" (#35143)

This reverts commit 94c80ff.

Author: mdo

scss/_buttons.scss

@@ -4,7 +4,6 @@
 
 .btn {
   display: inline-block;
-  align-self: center;
   font-family: $btn-font-family;
   font-weight: $btn-font-weight;
   line-height: $btn-line-height;