Bug 1555935 - Remove some unnecessary use of FreeOp where plain old js_delete will do r=pbone?

Carrying on from the last patch, there is one place where we use FreeOp unnecessarily when we're not finalizing.

Depends on D33276

Differential Revision: https://phabricator.services.mozilla.com/D33313

--HG--
extra : moz-landing-system : lando