[runtime] Simplify code to use existing local variable (#15441)

src/ObjCRuntime/Class.cs

@@ -292,7 +292,7 @@ unsafe static bool CompareTokenReference (string? asm_name, int mod_token, int t
 			if ((token_reference & 0x1) == 0x1) {
 				// full token reference
 				var idx = (int) (token_reference >> 1);
-				var entry = Runtime.options->RegistrationMap->full_token_references + (IntPtr.Size + 8) * idx;
+				var entry = map->full_token_references + (IntPtr.Size + 8) * idx;
 				// first compare what's most likely to fail (the type's metadata token)
 				var token = (uint) Marshal.ReadInt32 (entry + IntPtr.Size + 4);
 				type_token |= 0x02000000 /* TypeDef - the token type is explicit in the full token reference, but not present in the type_token argument, so we have to add it before comparing */;