Remove dead code in VisiblePosition, VisibleSelection, VisibleUnits.{cpp,h}, and markup.{cpp,h}.

This CL removes the followings because they are not used:
- VisiblePosition::getInlineBoxAndOffset(TextDirection, InlineBox*&, int&)
- endVisiblePosition
- VisibleSelection::isAll
- isStartOfWord
- inSameDocument
- createFragmentFromNodes
- createFullMarkup

BUG=none
TEST=none
R=yosin@chromium.org

Review URL: https://codereview.chromium.org/139933006

git-svn-id: svn://svn.chromium.org/blink/trunk@165380 bbb929c8-8fbe-4397-9dbb-9b2b20218538