Remove dead code in Source/core/editing/.

This CL removes the following unused code:
- ApplyStyleCommand::nodeFullyUnselected
- CharacterIterator::string
- CompositeEditCommand::insertLineBreak
- CompositeEditCommand::isCreateLinkCommand and an override
- CompositeEditCommand::shouldRetainAutocorrectionIndicator and an override
- DeleteSelectionCommand::calculateEndingPosition declaration
- DeleteSelectionCommand::insertPlaceholderForAncestorBlockContent declaration
- EditingCommand::isEditCommandComposition
- EditingStyle::EditingStyle(const CSSStyleDeclaration*)
- EditingStyle::create(const CSSStyleDeclaration*)
- EditingStyle::setStyle
- EditingStyle::shouldUseFixedDefaultFontSize
- IndentOutdentCommand::indentRegion declaration
- InsertLineBreakCommand::insertNodeAfterPosition
- InsertLineBreakCommand::insertNodeBeforePosition
- InsertListCommand::insertList
- InsertTextCommand::InsertTextCommand(Document&, const String&, PassRefPtr<TextInsertionMarkerSupplier>)
- InsertTextCommand::create(Document&, const String&, PassRefPtr<TextInsertionMarkerSupplier>)
- InsertTextCommand::deleteCharacter declaration
- InsertTextCommand::m_markerSupplier
- ReplaceSelectionCommand::handlePasteAsQuotationNode declaration
- SetSelectionCommand class
- SpellCheckRequester::isStarted
- SpellChecker::misspelledWordAtCaretOrRange
- TextInsertionMarkerSupplier class
- TypingCommand::smartDelete
- WordAwareIterator::range
- positionOutsideTabSpan in htmlediting.{cpp,h}

BUG=none
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/144233007

git-svn-id: svn://svn.chromium.org/blink/trunk@165542 bbb929c8-8fbe-4397-9dbb-9b2b20218538