[Code Cleanup] Remove two lines that was added for an experiment

This experiment is already done, so remove these two lines.
As long as all bots are green, this should be saefe.

TBR=pdr@chromium.org

Bug: None
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie27fa10cdde06cb9938cb0d7b5d55ad6fe9946fd
Reviewed-on: https://chromium-review.googlesource.com/1219782Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590338}

third_party/blink/renderer/core/paint/compositing/compositing_reason_finder.cc

@@ -217,10 +217,6 @@ CompositingReasons CompositingReasonFinder::CompositingReasonsForAnimation(
     reasons |= CompositingReason::kActiveFilterAnimation;
   if (RequiresCompositingForBackdropFilterAnimation(style))
     reasons |= CompositingReason::kActiveBackdropFilterAnimation;
-  // TODO(crbug.com/754471): remove the next two lines when the experiment is
-  // completed.
-  if (!style.ShouldCompositeForCurrentAnimations())
-    reasons = CompositingReason::kNone;
   return reasons;
 }