Use single file for extends (#3216)

* Use single file for extends

* Update changelog

sass/utilities/controls.sass

@@ -1,4 +1,4 @@
-@import "derived-variables.sass"
+@import "derived-variables"
 
 $control-radius: $radius !default
 $control-radius-small: $radius-small !default
@@ -39,9 +39,6 @@ $control-padding-horizontal: calc(0.75em - #{$control-border-width}) !default
   fieldset[disabled] &
     cursor: not-allowed
 
-%control
-  +control
-
 // The controls sizes use mixins so they can be used at different breakpoints
 =control-small
   border-radius: $control-radius-small