Remove the datetime keywords ABSTIME and RELTIME, which we'd been treating as

Remove the datetime keywords ABSTIME and RELTIME, which we'd been treating as
noise words for the last twelve years, for compatibility with Berkeley-era
output formatting of the special INVALID values for those datatypes.
Considering that the datatypes themselves have been deprecated for awhile,
this is taking backwards compatibility a little far.  Per gripe from Josh
Berkus.