Revert 0f5ca02f53

0f5ca02f53 introduces 3 new keywords.  It appears to be too much for relatively
small feature.  Given now we past feature freeze, it's already late for
discussion of the new syntax.  So, revert.

Discussion: https://postgr.es/m/28209.1586294824%40sss.pgh.pa.us

src/backend/utils/adt/misc.c

@@ -372,6 +372,8 @@ pg_sleep(PG_FUNCTION_ARGS)
 	 * less than the specified time when WaitLatch is terminated early by a
 	 * non-query-canceling signal such as SIGHUP.
 	 */
+#define GetNowFloat()	((float8) GetCurrentTimestamp() / 1000000.0)
+
 	endtime = GetNowFloat() + secs;
 
 	for (;;)