Remove deprecated methods from WebBlobRegistry.h

The removed methods were marked as deprecatd in the CL below. They do
not appear to be implemented or used anymore.
https://chromiumcodereview.appspot.com/23703003

BUG=

Review URL: https://codereview.chromium.org/363393003

git-svn-id: svn://svn.chromium.org/blink/trunk@177530 bbb929c8-8fbe-4397-9dbb-9b2b20218538