Anyway, this saves about 3KB in fmgrtab.o, without any downside that I can see. If someone accidentally called an aggregate as a normal function, they'd now get a different error message, namely "internal function "aggregate_dummy" is not in internal lookup table" instead of "aggregate function NNN called as normal function". That doesn't really seem like a problem.
Speaking as somebody who sometimes does really dumb things, I don’t like this change in error message. The current message clearly identifies the problem; the new message makes it look like there is a bug in Postgres.