Remove defunct `background-image: none` Firefox-on-Android workaround

Mozilla fixed the bug circa Firefox 33-35:
    https://bugzilla.mozilla.org/show_bug.cgi?id=763671
Firefox 34 is the current version.
Firefox 35 should be current by the time Bootstrap v4 is released.

scss/_buttons.scss

@@ -16,7 +16,6 @@
   touch-action: manipulation;
   cursor: pointer;
   user-select: none;
-  background-image: none; // Reset unusual Firefox-on-Android default style; see https://github.com/necolas/normalize.css/issues/214
   border: $border-width solid transparent;
   @include button-size($padding-base-vertical, $padding-base-horizontal, $font-size-base, $line-height-base, $border-radius-base);
   transition: all .2s ease-in-out;

scss/_forms.scss

@@ -122,7 +122,6 @@ output {
   line-height: $line-height-base;
   color: $input-color;
   background-color: $input-bg;
-  background-image: none; // Reset unusual Firefox-on-Android default style; see https://github.com/necolas/normalize.css/issues/214
   border: 1px solid $input-border;
   @include border-radius($input-border-radius);
   @include box-shadow(inset 0 1px 1px rgba(0,0,0,.075));