remove pgis_abs dummy type

[pramsey]

Since we no longer require a dummy type in order to drive the aggregation machinery, remove the pgis_abs type and replace with the 'internal' type.

[pramsey]

In 16443:

Remove the pgis_abs dummy type and replace with use of 'internal' type.
Closes #4035

[dbaston]

Does Postgres care about the size of an internal state type? I recall that we had to adjust pg_type when increasing the size of pgis_abs in the past.

[pramsey]

I don't know the answer to that, dbaston. I kept the pgis_abs struct, and the 'internal' type seems to just pass a pointer, so once the data which used to be in the pgis_abs type and was passed by value was correctly placed in the aggregate memory context everything worked again like magic.

[pramsey]

In 16446:

Annotate changed function signatures for pgis_abs removal
References #4035

[pramsey]

In 16447:

Drop unused functions for pgis_abs during upgrades
References #4035