remove the `float` type

It is simply defined as `f64` across every platform right now.

A best-effort type with the highest precision offered by the CPU would
be strange, because results would vary across platforms. A use case for
this hasn't been presented in the RFC.

If there was a use case, this could be implemented as simply a type
alias or a struct thanks to `#[cfg(...)]`.

If 32-bit precision is enough, `f32` will be faster than `f64` due to
halving the needed cache space and fitting twice as many in an SSE/AVX
register. The same thing will apply to using `f64` even if an `f128`
type is available.

Closes #6592

The mailing list thread, for reference:

https://mail.mozilla.org/pipermail/rust-dev/2013-July/004632.html