bug 1552310 - use the correct field to delete preloaded certificates that have been removed from the preload list r=jcj,KevinJacobs

The initial implementation made some incorrect assumptions about the data that
was in our data set and used the wrong field to identify the certificates to
delete when they are removed from our preload list. Now that the data set has
the expected field (the hash of the whole certificate), we can use it instead.

Differential Revision: https://phabricator.services.mozilla.com/D32380

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extra : moz-landing-system : lando