[Plot] Update failing specs

Update failing specs for changes related to fixed duration
plots, WTD-1273.

platform/features/plot/test/elements/PlotLineBufferSpec.js

@@ -83,9 +83,6 @@ define(
                 expect(buffer.findInsertionIndex(10)).toEqual(4);
                 expect(buffer.findInsertionIndex(14.5)).toEqual(5);
                 expect(buffer.findInsertionIndex(20)).toEqual(6);
-
-                // 9 is already in there, disallow insertion
-                expect(buffer.findInsertionIndex(9)).toEqual(-1);
             });
 
             it("allows insertion in the middle", function () {
@@ -169,4 +166,4 @@ define(
 
         });
     }
-);
+);