_progress.scss: remove unnecessary @-webkit-keyframes

Autoprefixer already takes care of this for us.

dist/css/bootstrap.css

@@ -3994,7 +3994,6 @@ a.badge:hover, a.badge:focus {
     background-position: 0 0;
   }
 }
-
 @-o-keyframes progress-bar-stripes {
   from {
     background-position: 1 0;
@@ -4004,7 +4003,6 @@ a.badge:hover, a.badge:focus {
     background-position: 0 0;
   }
 }
-
 @keyframes progress-bar-stripes {
   from {
     background-position: 1 0;

scss/_progress.scss

@@ -9,11 +9,6 @@
 // Progress animations
 //
 
-@-webkit-keyframes progress-bar-stripes {
-  from  { background-position: $line-height-computed 0; }
-  to    { background-position: 0 0; }
-}
-
 @keyframes progress-bar-stripes {
   from  { background-position: $line-height-computed 0; }
   to    { background-position: 0 0; }