tpm xen: Remove bogus tpm_chip_unregister

tpm_chip_unregister can only be called after tpm_chip_register.
devm manages the allocation so no unwind is needed here.

Cc: stable@vger.kernel.org
Fixes: afb5abc2 ("tpm: two-phase chip management functions")
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>