t/test-full.proto: revert 'option allow_alias = true;'

this option is only supported by the upstream protobuf compiler starting
with version 2.5.0. this version is not yet widely available in the
debian/ubuntu repositories, and we would like to avoid breaking the
build on those platforms with the distribution version of protobuf
installed, so revert the following commits:

 - 5ee9c03478ea13bba03e7d7edacf723f324200c2
 - 84e41e7329f1f0fc09b41ee96e17b28a792cefcf