Remove NewOrExistingInterval logic from SVGSMILElement

The NewOrExistingInterval logic is no longer used and can be removed. No
new tests as this is just a refactoring.

Review URL: https://codereview.chromium.org/90843002

git-svn-id: svn://svn.chromium.org/blink/trunk@162762 bbb929c8-8fbe-4397-9dbb-9b2b20218538