[spell] Remove unused spellcheck preference.

This patch removes the unused, not synced preference:

    spellcheck.confirm_dialog_shown

BUG=331291

Review URL: https://codereview.chromium.org/105303005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242953 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242953