remove the float type

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

A use case hasn't been presented for a `float` type defined as the
highest precision floating point type implemented in hardware on the
platform. Performance-wise, using the smallest precision correct for the
use case greatly saves on cache space and allows for fitting more
numbers into SSE/AVX registers.

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

Closes #6592

The mailing list thread, for reference:

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

src/librustdoc/html/format.rs

@@ -233,7 +233,6 @@ impl fmt::Default for clean::Type {
                     ast::ty_uint(ast::ty_u16) => "u16",
                     ast::ty_uint(ast::ty_u32) => "u32",
                     ast::ty_uint(ast::ty_u64) => "u64",
-                    ast::ty_float(ast::ty_f) => "float",
                     ast::ty_float(ast::ty_f32) => "f32",
                     ast::ty_float(ast::ty_f64) => "f64",
                     ast::ty_str => "str",