Remove framework gating

PortableExecutable/COFFStringTable.cs

@@ -20,10 +20,6 @@
         /// Following the size are null-terminated strings that are pointed to by symbols
         /// in the COFF symbol table.
         /// </summary>
-#if NET48
-        public string[] Strings { get; set; }
-#else
         public string[]? Strings { get; set; }
-#endif
     }
 }