Remove all uses of selector &

If we want to support namespaced import:

```scss
.twbs {
  @import 'bootstrap';
}
```

We cannot use `selector &`, see sass/sass#1817.

`fieldset[disabled] &` is not needed as we no longer support IE8.

scss/mixins/_buttons.scss

@@ -45,8 +45,7 @@
   }
 
   &.disabled,
-  &:disabled,
-  fieldset[disabled] & {
+  &:disabled {
     &:focus,
     &.focus {
       background-color: $background;
@@ -81,8 +80,7 @@
   }
 
   &.disabled,
-  &:disabled,
-  fieldset[disabled] & {
+  &:disabled {
     &:focus,
     &.focus {
       border-color: lighten($color, 20%);