Revert "gtlscertificate: Add certificate-bytes and private-key-bytes props"

 * In order to add contstruct properties to an abstract base
   calls, and retain ABI stability, the base class must add a
   default implementation of those properties.
 * We cannot add a default implementation of certificate-bytes
   or private-key-bytes since certificate and private-key properties
   are writable on construct-only.

This reverts commit 541c9858.

https://bugzilla.gnome.org/show_bug.cgi?id=682081