pgsql: Remove useless entries for aggregate functions from fmgrtab.c.

Remove useless entries for aggregate functions from fmgrtab.c.

Gen_fmgrtab.pl treated aggregate functions the same as other built-in
functions, which is wasteful because there is no real need to have
entries for them in the fmgr_builtins[] table. Suppressing those
entries saves about 3KB in the compiled table on my machine; which
is not a lot but it's not nothing either, considering that that
table is pretty "hot". The only outside code change needed is
that ExecInitWindowAgg() can't be allowed to call fmgr_info_cxt()
on a plain aggregate function. But that saves a few cycles anyway.

Having done that, the aggregate_dummy() function is unreferenced
and might as well be dropped. Using "aggregate_dummy" as the prosrc
value for an aggregate is now just a documentation convention not
something that matters. There was some discussion of using NULL
instead to save a few bytes in pg_proc, but we'd have to remove
prosrc's BKI_FORCE_NOT_NULL marking which doesn't seem a great idea.
Anyway, it's possible there's client-side code that expects to
see "aggregate_dummy" there, so I'm loath to change it without a
strong reason.

Discussion: https://postgr.es/m/533989.1604263665@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f21636e5d5b8394ed076e18ddc5f4ba710c69c99

Modified Files
--------------
src/backend/catalog/pg_aggregate.c | 2 +-
src/backend/executor/nodeAgg.c | 18 ------------------
src/backend/executor/nodeWindowAgg.c | 10 +++++-----
src/backend/utils/Gen_fmgrtab.pl | 9 +++++++--
4 files changed, 13 insertions(+), 26 deletions(-)