remove dateString from Item union (#198)

It has been unused for a long time. Taking advantage of 2.x to remove it.

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/inc/qcbor/qcbor_decode.h b/inc/qcbor/qcbor_decode.h
index a7b85ee..f40200a 100644
--- a/inc/qcbor/qcbor_decode.h
+++ b/inc/qcbor/qcbor_decode.h
@@ -470,9 +470,7 @@
       /** The value for @c uDataType @ref QCBOR_TYPE_DAYS_EPOCH -- the
        *  number of days before or after Jan 1, 1970. */
       int64_t     epochDays;
-      /** No longer used. Was the value for @ref QCBOR_TYPE_DATE_STRING,
-       * but now that value is in @c string. This will be removed in QCBOR 2.0. */
-      UsefulBufC  dateString;
+
       /** The value for @c uDataType @ref QCBOR_TYPE_POSBIGNUM and
        * @ref QCBOR_TYPE_NEGBIGNUM.  */
       UsefulBufC  bigNum;