Remove nullability support for `{Owned,Unowned}{Fd,CFile}` and
`{Fd,CFile,Digester,DigesterBase}Handle`.

Sadly, for `Owned` classes they are incompatible with `Open()` and `Close()`
given that nullability analysis does not know anything about these functions:

* The caller has to assert that the target is not null after a successful
  `Open()`, if it is subsequently passed to a function which requires
  a non-null parameter.

* If `Open()` is called on a member variable, the call to `Open()` itself is
  flagged as reading a null value. This cannot be silenced with an assertion
  because it is indeed null and `Open()` is valid despite this.

  I don't know why this does not apply to a local variable.

* `Close()` makes the object null in a way which is opaque to nullability
  analysis.

An explicit `absl_nullability_unknown` would silence that. Still, deciding when
to apply that, and when to apply an intended nullability annotation, would be
too subtle.

`Unowned` classes should be analogous to avoid confusion.

For `Handle` classes, it is doubtful that nullability annotations would be
useful, because in clients they are mostly used as template arguments, namely
in `Dependency<...Handle, ...>` and `Any<...Handle>`.

PiperOrigin-RevId: 806268974