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/libsyntax/ast_util.rs

@@ -190,7 +190,7 @@ pub fn uint_ty_max(t: uint_ty) -> u64 {
 }
 
 pub fn float_ty_to_str(t: float_ty) -> ~str {
-    match t { ty_f => ~"f", ty_f32 => ~"f32", ty_f64 => ~"f64" }
+    match t { ty_f32 => ~"f32", ty_f64 => ~"f64" }
 }
 
 pub fn is_call_expr(e: @Expr) -> bool {