Editorial: Remove unused algorithm steps

Follow up from #2822. Once again thanks to Anba for spotting this.

polyfill/lib/ecmascript.mjs

@@ -3898,15 +3898,7 @@ export function DifferenceISODateTime(
   return { years, months, weeks, days, norm: timeDuration };
 }
 
-export function DifferenceZonedDateTime(
-  ns1,
-  ns2,
-  timeZoneRec,
-  calendarRec,
-  largestUnit,
-  options,
-  precalculatedDtStart = undefined
-) {
+export function DifferenceZonedDateTime(ns1, ns2, timeZoneRec, calendarRec, largestUnit, options, dtStart) {
   // getOffsetNanosecondsFor and getPossibleInstantsFor must be looked up
   // dateAdd must be looked up if the instants are not identical (and the date
   // difference has no years, months, or weeks, which can't be determined)
@@ -3926,9 +3918,7 @@ export function DifferenceZonedDateTime(
 
   // Convert start/end instants to datetimes
   const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
-  const start = new TemporalInstant(ns1);
   const end = new TemporalInstant(ns2);
-  const dtStart = precalculatedDtStart ?? GetPlainDateTimeFor(timeZoneRec, start, calendarRec.receiver);
   const dtEnd = GetPlainDateTimeFor(timeZoneRec, end, calendarRec.receiver);
 
   // Simulate moving ns1 as many years/months/weeks/days as possible without

spec/zoneddatetime.html

@@ -1422,7 +1422,7 @@ <h1>
           _calendarRec_: a Calendar Methods Record,
           _largestUnit_: a String,
           _options_: an ordinary Object for which the value of the [[Prototype]] internal slot is *null* and every property is a data property,
-          _precalculatedPlainDateTime_: a Temporal.PlainDateTime,
+          _startDateTime_: a Temporal.PlainDateTime,
         ): either a normal completion containing a Normalized Duration Record, or a throw completion
       </h1>
       <dl class="header">
@@ -1435,11 +1435,6 @@ <h1>
       <emu-alg>
         1. If _ns1_ is _ns2_, then
           1. Return ! CreateNormalizedDurationRecord(0, 0, 0, 0, ZeroTimeDuration()).
-        1. If _precalculatedPlainDateTime_ is *undefined*, then
-          1. Let _startInstant_ be ! CreateTemporalInstant(_ns1_).
-          1. Let _startDateTime_ be ? GetPlainDateTimeFor(_timeZoneRec_, _startInstant_, _calendarRec_.[[Receiver]]).
-        1. Else,
-          1. Let _startDateTime_ be _precalculatedPlainDateTime_.
         1. Let _endInstant_ be ! CreateTemporalInstant(_ns2_).
         1. Let _endDateTime_ be ? GetPlainDateTimeFor(_timeZoneRec_, _endInstant_, _calendarRec_.[[Receiver]]).
         1. If _ns2_ - _ns1_ &lt; 0, let _sign_ be -1; else let _sign_ be 1.