Remove private isnan function with only one caller

Author: wilzbach

src/object.d

@@ -927,7 +927,7 @@ nothrow @safe @nogc unittest
             destroy!false(a);
             assert(a == 42);
             destroy(a);
-            assert(isnan(a));
+            assert(a != a); // isnan
         }
     }
 
@@ -3632,14 +3632,6 @@ unittest
     assert(postblitRecurseOrder == order);
 }
 
-version (unittest)
-{
-    private bool isnan(float x)
-    {
-        return x != x;
-    }
-}
-
 private
 {
     extern (C) void _d_arrayshrinkfit(const TypeInfo ti, void[] arr) nothrow;