Clear key_method in ssl_cert_clear_certs.

Since it resets leaf, private key, and chain, it makes sense to also
clear custom key method tables.

Change-Id: If511b8f15a44674c31d068d36984e9189c5a9071
Reviewed-on: https://boringssl-review.googlesource.com/5356
Reviewed-by: Adam Langley <agl@google.com>