Use single file for extends (#3216)

* Use single file for extends

* Update changelog

sass/utilities/mixins.sass

@@ -161,9 +161,6 @@
   -ms-user-select: none
   user-select: none
 
-%unselectable
-  +unselectable
-
 =arrow($color: transparent)
   border: 3px solid $color
   border-radius: 2px
@@ -180,18 +177,12 @@
   transform-origin: center
   width: 0.625em
 
-%arrow
-  +arrow
-
 =block($spacing: $block-spacing)
   &:not(:last-child)
     margin-bottom: $spacing
 
-%block
-  +block
-
 =delete
-  @extend %unselectable
+  +unselectable
   -moz-appearance: none
   -webkit-appearance: none
   background-color: bulmaRgba($scheme-invert, 0.2)
@@ -256,9 +247,6 @@
     min-width: 32px
     width: 32px
 
-%delete
-  +delete
-
 =loader
   animation: spinAround 500ms infinite linear
   border: 2px solid $grey-lighter
@@ -271,9 +259,6 @@
   position: relative
   width: 1em
 
-%loader
-  +loader
-
 =overlay($offset: 0)
   bottom: $offset
   left: $offset
@@ -281,5 +266,3 @@
   right: $offset
   top: $offset
 
-%overlay
-  +overlay