Remove unused keyNeeded() interface

Original (https://codereview.chromium.org/129793008/) broke the Android build,
but that has now been updated (https://codereview.chromium.org/146693003/).
Test from the original CL is being updated seperately.

BUG=224786
TEST=EME content tests pass

Review URL: https://codereview.chromium.org/138233004

git-svn-id: svn://svn.chromium.org/blink/trunk@166102 bbb929c8-8fbe-4397-9dbb-9b2b20218538