Remove psql's ancient hack that suppressed functions taking or returning

cstring from the output of \df.  Now that the default behavior is to
exclude all system functions, the de-cluttering rationale for this behavior
seems pretty weak; and it was always quite confusing/unhelpful if you were
actually looking for I/O functions.  (Not to mention if you were looking
for encoding converters or other cases that might take or return cstring.)