Remove Rng.choose(), rename Rng.choose_option() to .choose()

Rng.choose() is used so rarely that it doesn't necessitate having two
methods, especially since the failing, non-option variant also requires
Clone.

[breaking-change]
auto merge of #14319 : kballard/rust/rename_rng_choose_option, r=alexcrichton

Rng.choose() is used so rarely that it doesn't necessitate having two
methods, especially since the failing, non-option variant also requires
Clone.

[breaking-change]