Remove progress bar striped @color parameters; use default transparent white.

* If the @color parameter is not removed, the stripes are the same color as the element background color. In this case, the stripes do not appear.

less/mixins.less

@@ -496,7 +496,7 @@
 .progress-bar-variant(@color) {
   background-color: @color;
   .progress-striped & {
-    #gradient > .striped(@color);
+    #gradient > .striped();
   }
 }
 

less/progress-bars.less

@@ -60,7 +60,7 @@
 
 // Striped bars
 .progress-striped .progress-bar {
-  #gradient > .striped(@progress-bar-bg);
+  #gradient > .striped();
   background-size: 40px 40px;
 }