tpm-backend: Remove unneeded member variable from backend class

tpm-backend: Remove unneeded member variable from backend class

TPMDriverOps inside TPMBackend is not required, as it is supposed to be a class
member. The only possible reason for keeping in TPMBackend was, to get the
backend type in tpm.c where dedicated backend api, tpm_backend_get_type() is
present.

Signed-off-by: Amarnath Valluri <amarnath.valluri@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>