[SVG] Remove unneeded hashmap lookup in SVGDocumentExtensions::removeResource()

No reason to check whether the id is present before calling remove(), it
just adds an extra lookup if it is.

R=pdr@chromium.org,schenney@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23627010

git-svn-id: svn://svn.chromium.org/blink/trunk@157719 bbb929c8-8fbe-4397-9dbb-9b2b20218538