Remove unprefixing logic from interpolation test helper

This was helpful to allow a number of tests to run under other browsers, but
as we're starting to unprefix some properties we will add additional tests
instead.

Review URL: https://codereview.chromium.org/212073002

git-svn-id: svn://svn.chromium.org/blink/trunk@170028 bbb929c8-8fbe-4397-9dbb-9b2b20218538