sagemathgh-37822: Fix bug in TensorField.apply_map
    
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In Sage 10.4.beta3 we have

```
sage: E.<x,y> = EuclideanSpace()
sage: a = var('a')
sage: v = E.vector_field(x, a*y)
sage: v.divergence().expr()  # OK
a + 1
sage: v.apply_map(lambda t: t.subs(a=-1))
sage: v.display()
x e_x - y e_y
sage: v.divergence().expr()  # wrong!
a + 1
```
The last output should be 0.

This error occurs because the method `_del_derived ` is not invoked to
reset derived quantities relative to the components of a tensor field
that are modified by `apply_map`.  In the above example, the
differentials of the components, which are invoked in computing the
divergence, are cached and not deleted by `apply_map`.
This is corrected here.
    
URL: sagemath#37822
Reported by: Eric Gourgoulhon
Reviewer(s): Matthias Köppe