Cleanup: Remove keygenMenu*GradeKeySize functions.

This is a leftover of r158372.

BUG=298020
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/24734002

git-svn-id: svn://svn.chromium.org/blink/trunk@158420 bbb929c8-8fbe-4397-9dbb-9b2b20218538