Make saturating u128 -> f32 casts the default behavior

Make saturating u128 -> f32 casts the default behavior

... rather than being gated by -Z saturating-float-casts.
There are several reasons for this:

1. Const eval already implements this behavior.
2. Unlike with float->int casts, this behavior is uncontroversially the
right behavior and it is not as performance critical. Thus there is no
particular need to make the bug fix for u128->f32 casts opt-in.
3. Having two orthogonal features under one flag is silly, and never
should have happened in the first place.
4. Benchmarking float->int casts with the -Z flag should not pick up
performance changes due to the u128->f32 casts (assuming there are any).

Fixes #41799