Remove unused input-size mixin (twbs#21971)

scss/mixins/_forms.scss

@@ -53,29 +53,3 @@
     @include box-shadow($input-box-shadow-focus);
   }
 }
-
-// Form control sizing
-//
-// Relative text size, padding, and border-radii changes for form controls. For
-// horizontal sizing, wrap controls in the predefined grid classes. `<select>`
-// element gets special love because it's special, and that's a fact!
-
-@mixin input-size($parent, $input-height, $padding-y, $padding-x, $font-size, $line-height, $border-radius) {
-  #{$parent} {
-    height: $input-height;
-    padding: $padding-y $padding-x;
-    font-size: $font-size;
-    line-height: $line-height;
-    @include border-radius($border-radius);
-  }
-
-  select#{$parent} {
-    height: $input-height;
-    line-height: $input-height;
-  }
-
-  textarea#{$parent},
-  select[multiple]#{$parent} {
-    height: auto;
-  }
-}