gpg: don't clear the session after PIN is entered

This would allow single PIN entry when running multiple GPG commands.

trezor_agent/gpg/encode.py

@@ -52,7 +52,6 @@ class HardwareSigner(object):
 
     def close(self):
         """Close the connection to the device."""
-        self.client_wrapper.connection.clear_session()
         self.client_wrapper.connection.close()