Remove "15Z" to for PlainTime.from test

"15Z" will throw RangeError because Z match 
UTCDesignator => 
TimeZoneUTCOffset => 
TimeZoneUTCOffset TimeZoneBracketedAnnotationopt => 
TimeZoneopt 

and "15" match
1 DecimalDigit =>
Hour =>
TimeHour  =>
TimeSpec 

so "15Z" match
TimeSpec TimeZoneopt =>
TimeSpecWithOptionalTimeZoneNotAmbiguous =>
TimeSpecWithOptionalTimeZoneNotAmbiguous Calendaropt =>
CalendarTime  =>
TemporalTimeString 

Therefore 
13.37 ParseTemporalTimeString ( isoString )

https://tc39.es/proposal-temporal/#sec-temporal-parsetemporaltimestring
will succeess in 2. If parseResult is a List of errors, throw a RangeError exception.
but the in step 3
3. If parseResult contains a UTCDesignator Parse Node, throw a RangeError exception.

the "Z" is UTCDesignator so it will throw RangeError.

test/staging/Temporal/Regex/old/plaintime.js

@@ -45,7 +45,6 @@ function generateTest(dateTimeString, zoneString) {
   "[Europe/Vienna]",
   "+01:00[Custom/Vienna]",
   "-04:00",
-  "Z",
   ""
 ].forEach(zoneStr => test(`15${ zoneStr }`, [15]));
 [