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/test/debug-info/generic-static-method-on-struct-and-enum.rs

@@ -46,7 +46,7 @@ impl Struct {
 enum Enum {
     Variant1 { x: int },
     Variant2,
-    Variant3(float, int, char),
+    Variant3(f64, int, char),
 }
 
 impl Enum {