Drop background-clip and background-size mixins since they have no prefixes anyway

dist/css/bootstrap.css

@@ -4003,8 +4003,6 @@ button.close {
   border-radius: 6px;
   -webkit-box-shadow: 0 5px 10px rgba(0, 0, 0, 0.2);
           box-shadow: 0 5px 10px rgba(0, 0, 0, 0.2);
-  -webkit-background-clip: padding-box;
-     -moz-background-clip: padding;
   background-clip: padding-box;
 }
 

less/dropdowns.less

@@ -38,7 +38,7 @@
   border: 1px solid @dropdown-border;
   border-radius: @border-radius-base;
   .box-shadow(0 6px 12px rgba(0,0,0,.175));
-  .background-clip(padding-box);
+  background-clip: padding-box;
 
   // Aligns the dropdown menu to right
   &.pull-right {

less/mixins.less

@@ -163,16 +163,6 @@
           backface-visibility: @visibility;
 }
 
-// Background clipping
-.background-clip(@clip) {
-  background-clip: @clip;
-}
-
-// Background sizing
-.background-size(@size) {
-  background-size: @size;
-}
-
 // Box sizing
 .box-sizing(@boxmodel) {
   -webkit-box-sizing: @boxmodel;

less/modals.less

@@ -49,7 +49,7 @@
   border: 1px solid @modal-content-border-color;
   border-radius: @border-radius-large;
   .box-shadow(0 3px 9px rgba(0,0,0,.5));
-  .background-clip(padding-box);
+  background-clip: padding-box;
   // Remove focus outline from opened modal
   outline: none;
 }

less/popovers.less

@@ -13,8 +13,6 @@
   padding: 1px;
   text-align: left; // Reset given new insertion method
   background-color: @popover-bg;
-  -webkit-background-clip: padding-box;
-     -moz-background-clip: padding;
   background-clip: padding-box;
   border: 1px solid @popover-fallback-border-color;
   border: 1px solid @popover-border-color;

less/progress-bars.less

@@ -61,7 +61,7 @@
 // Striped bars
 .progress-striped .progress-bar {
   #gradient > .striped(@progress-bar-bg);
-  .background-size(40px 40px);
+  background-size: 40px 40px;
 }
 
 // Call animation for the active one