Clean up Editor class

This patch cleans Editor class up by
- removing unused member functions rangeForPoint(), removedAnchor(), setRemovedAnchor(), simplifyMarkup().
- expanding isContinuousSpellCheckingEnabled() and removes it
- making rangeOfString() private

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/63363009

git-svn-id: svn://svn.chromium.org/blink/trunk@162328 bbb929c8-8fbe-4397-9dbb-9b2b20218538