Remove dead code in DecodeInterval()

This commit removes some dead code related to the unit type RESERV,
whose last use has been removed from the unit lookup table used for
intervals ("deltatktbl" in datetime.c) in 666cbae16da4.  Before that,
RESERV was used as an equivalent of "invalid", but that's now
unreachable.

Author: Joseph Koshakow
Reviewed-by: Jacob Champion, Gurjeet Singh, Reid Thompson
Discussion: https://postgr.es/m/CAAvxfHd-yNO+XYnUxL=GaNZ1n+eE0V-oE0+-cC1jdjdU0KS3iw@mail.gmail.com

src/backend/utils/adt/datetime.c

@@ -3582,11 +3582,6 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 						type = uval;
 						break;
 
-					case RESERV:
-						tmask = (DTK_DATE_M | DTK_TIME_M);
-						*dtype = uval;
-						break;
-
 					default:
 						return DTERR_BAD_FORMAT;
 				}