Remove WEBCRYPTO_HMAC_KEY_HAS_LENGTH.

This is no longer needed since >1 month ago:
https://codereview.chromium.org/205583003

BUG=245025

Review URL: https://codereview.chromium.org/291583006

git-svn-id: svn://svn.chromium.org/blink/trunk@174495 bbb929c8-8fbe-4397-9dbb-9b2b20218538