Remove unused feature HTMLAppletElementLegacyCall

Intent to Remove:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/MuxvaXiyBxU

BUG=339919

Review URL: https://codereview.chromium.org/719013002

git-svn-id: svn://svn.chromium.org/blink/trunk@185204 bbb929c8-8fbe-4397-9dbb-9b2b20218538