common: Allow for GCM decryption in de-vs mode.

* common/compliance.c (gnupg_cipher_is_allowed): Allow GCM for gpgsm
in decrypt mode.
* tests/cms/samplemsgs/pwri-sample.gcm.p7m: Remove duplicated authtag

We allow GCM in de-vs mode for decryption although this has not been
evaluation. It is decryption and thus no serious harm may happen.