Remove contentType check in encrypted-media-needkey.html.

This check will be restored after the 'needkey' is renamed to 'encrypted'.

BUG=224786

Review URL: https://codereview.chromium.org/620753003

git-svn-id: svn://svn.chromium.org/blink/trunk@183020 bbb929c8-8fbe-4397-9dbb-9b2b20218538