Remove some dead code from AXObjectCache

Remove some dead code from AXObjectCache. The textMarkerDataForVisiblePosition()
and visiblePositionForTextMarkerData() are defined but never used.

Review URL: https://codereview.chromium.org/238723010

git-svn-id: svn://svn.chromium.org/blink/trunk@171841 bbb929c8-8fbe-4397-9dbb-9b2b20218538