pgsql: Remove the long-obsolete homebrew dl*() functions for AIX, in

Log Message:
-----------
Remove the long-obsolete homebrew dl*() functions for AIX, in favor of just
using the system functions all the time.  (These files are now just copies
of the osf.* files.)  The homebrew functions were not getting used anyway
on AIX versions that have dlopen(), that is 4.3 and up, so they are not
needed on any AIX that is even remotely supported by the vendor anymore.
We'd have probably left them here anyway, except some questions were
raised about the copyright.

Modified Files:
--------------
    pgsql/src/backend/port/dynloader:
        aix.c (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/dynloader/aix.c?r1=1.20&r2=1.21)
        aix.h (r1.13 -> r1.14)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/dynloader/aix.h?r1=1.13&r2=1.14)

Removed Files:
-------------
    pgsql/src/backend/port/dynloader:
        README.dlfcn.aix
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/dynloader/README.dlfcn.aix)