Fix failing fullcopy test.

Fix failing fullcopy test.

The code deleted here was added in 63a425f0ef2b018f03eef9dd7486a7c1a9b2a390.
However, in my testing on PostgreSQL 9.2, the sequence replication tests in
t/10-fullcopy.t failed. I tried changing `defined` to `exists` but it did not
help. So I'm thinking there must be a better way to deal with sequence
attributes in older versions of PostgreSQL, though I don't know what it might
be.