Delete unnecessary functions in IBusPanelCandidateWindowHandlerInterface.

BUG=N/A

Review URL: https://codereview.chromium.org/88273003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237554 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237554