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(...)]`.

Performance-wise, if 32-bit precision is enough, `f32` will usually 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` if only 64-bit precision is required but an 128-bit type is
available.

Closes #6592

The mailing list thread, for reference:

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