Leftovers of random_in_unit_sphere

[whydoubt]

There are still a couple of mentions of random_in_unit_sphere in the books,
that should probably be switched out for random_unit_vector:

• For IOW, it's referenced when adding random_in_unit_disc.
• For TROYL, it's used as an example of a Las Vegas algorithm.
  random_in_unit_disc would work too, and would be a smaller code snippet.

[hollasch]

Ref #1606, #1616