Fixed declared but unused variable

"@progressPosition" is declared in the default theme but not used in the definition file, making it impossible to override.

src/definitions/modules/progress.less

@@ -138,7 +138,7 @@
 /* Percent Complete */
 .ui.progress .bar > .progress {
   white-space: nowrap;
-  position: absolute;
+  position: @progressPosition;
   width: @progressWidth;
   font-size: @progressSize;
   top: @progressTop;