Remove any mention of gender from docs

120ee51 removed the gender function and 0c49ffc removed the gender tests, so
it's about time to remove any mention of gender in the docs.

I'm assuming the preferred way for users at home to generate random genders is for them to use `random.array_element`

```js
Faker.random.array_element(['M', 'F']);
Faker.random.array_element(['Male', 'Female']);
```