remove AggDistribution and Agg->aggdistribution

remove AggDistribution and Agg->aggdistribution

As the plan is to rework the aggregate to use the infrastructure
from 9.6, the best idea seems to be to use the partial aggregate
stuff. Which means we can use the aggsplit field in Agg plan node,
and we don't need the custom aggdistribution.

Due to the upper-planner pathification, this should be at AggPath
anyway (but we already have the aggsplit there).

Furthermore, remove the custom plan construction from make_agg().
This was appropriate when the upper planner was not using paths,
but that's not true anymore. The idea is to construct the usual
Finalize+Partial aggregate nodes, possibly with a RemoteSubPath
node in between them (when pushdown is not possible).

I wonder how this will interact with the parallel query, as that
is also using the Partial node, but I suppose that should work
just fine. In the worst case we'll need to introduce something
like GatherCombine, collecting (and combining) partial results
on the data node before sending them to the coordinator.

Thanks to using Finalize/Partial Agg nodes, we can also remove
the changes from fix_upper_expr, or at least that's how it looks
to me.

At some point, bits of the removed code probably will need to be
restored, although most likely different place. That applies in
particular to:

(a) find_referenced_cols_walker(), which checks what part of the
aggregate we can push to the remote node (everything, partial
or nothing)

(b) the removed bits of make_agg() constructing the remote plan
(although most of this will hopefully be handled by the partial
aggregate infrastructure)