Remove PeerConnectionFactory's certificate generator and ref counted wrapper.
Clean-up.

The idea behind the factory having a certificate generator was that it would
reuse it with any peer connection it creates unless otherwise specified. This
generator was originally a DtlsIdentityStoreImpl store which preemptively
generated RSA-1024 in the background, giving any peer connection using RSA-1024
a head-start (generate before requesting). But now that 1) the store has been
replaced by a generator that does not do preemptive generation and 2) the
default is ECDSA, not RSA-1024, it is unnecessary for the factory to have this
code.

BUG=webrtc:5707, webrtc:5708
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/2021663002 .

Cr-Commit-Position: refs/heads/master@{#12993}