remove nullability annotation for Adjustment rule

Author: kkukshtel-unity

external/corefx-bugfix/src/Common/src/CoreLib/System/TimeZoneInfo.Unix.cs

@@ -1011,7 +1011,7 @@ private static void TZif_GenerateAdjustmentRule(ref int index, TimeSpan timeZone
 
                 // pulled in from this fix
                 // https://github.com/dotnet/runtime/pull/458/
-                AdjustmentRule? r = !string.IsNullOrEmpty(futureTransitionsPosixFormat) ?
+                AdjustmentRule r = !string.IsNullOrEmpty(futureTransitionsPosixFormat) ?
                     TZif_CreateAdjustmentRuleForPosixFormat(futureTransitionsPosixFormat, startTransitionDate, timeZoneBaseUtcOffset) :
                     null;
                 if (r == null)