Remove unused functions in LocalizedStrings.

There are no callsites of the following functions. We remove them.
 AXWebAreaText()
 AXLinkText()
 AXListMarkerText()
 AXImageMapText()
 AXHeadingText()
 AXFileUploadButtonText()

BUG=298020
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/25462002

git-svn-id: svn://svn.chromium.org/blink/trunk@158633 bbb929c8-8fbe-4397-9dbb-9b2b20218538