Removal of redundant function call in Editor::insertTextWithoutSendingTextEvent.

Minor cleanup.
'text' can never be empty as it's already been checked (line 830).

Review URL: https://codereview.chromium.org/73523006

git-svn-id: svn://svn.chromium.org/blink/trunk@162337 bbb929c8-8fbe-4397-9dbb-9b2b20218538