More removal of port dependent stuff

src/backend/port/BSD44_derived/Makefile

src/backend/port/BSD44_derived/README

src/backend/port/BSD44_derived/dl.c

src/backend/port/BSD44_derived/port-protos.h

src/backend/port/aix/Makefile

src/backend/port/aix/README.dlfcn

src/backend/port/aix/dlfcn.c

src/backend/port/aix/dlfcn.h

src/backend/port/aix/port-protos.h