Un-include access/heapam.h

I noticed that un-including access/heapam.h from the following files
leaves things just fine.

src/backend/commands/aggregatecmds.c
src/backend/commands/collationcmds.c
src/backend/commands/conversioncmds.c
src/backend/commands/lockcmds.c

It seems any calls into heapam.c that there used to be have since moved to
the corresponding backend/catalog/* files or elsewhere.

Attached a patch.

Thanks,
Amit