Remove an unused argument from SpellChecker::spellCheckOldSelection.

I'm not sure since when, anyway |newSelectedSentence| is no longer used.

BUG=

Review URL: https://codereview.chromium.org/350993002

git-svn-id: svn://svn.chromium.org/blink/trunk@177080 bbb929c8-8fbe-4397-9dbb-9b2b20218538