Fix compile errors

Author: angularsen

Samples/ConsoleApp-NetCore/Program.cs

@@ -1,7 +1,4 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Threading.Tasks;
 using UnitsNet;
 
 namespace ConsoleApp1

Samples/ConsoleApp-NetCore/Properties/AssemblyInfo.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 
 namespace UnitsNet.Samples.UnitConverter.Wpf
 {
@@ -14,7 +14,7 @@ public UnitListItem(object val)
             UnitEnumValue = val;
             UnitEnumValueInt = (int) val;
             UnitEnumType = val.GetType();
-            Abbreviation = UnitSystem.Default.GetDefaultAbbreviation(UnitEnumType, UnitEnumValueInt);
+            Abbreviation = UnitAbbreviationsCache.Default.GetDefaultAbbreviation(UnitEnumType, UnitEnumValueInt);
 
             Text = $"{val} [{Abbreviation}]";
         }
@@ -25,4 +25,4 @@ public UnitListItem(object val)
         public Type UnitEnumType { get; }
         public string Abbreviation { get; }
     }
-}
+}