Remove bogus optimization in RenderLayerCompositor::removeViewportConstrainedLayer

There's no reason to call |contains| on the HashSet before removing the entry.
Removing an item that doesn't exist in a HashSet is a nop. All the contains
call does is waste time with another hash lookup.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/269723010

git-svn-id: svn://svn.chromium.org/blink/trunk@173350 bbb929c8-8fbe-4397-9dbb-9b2b20218538