Remove enum -> float casts

[brson]

Fixed in #14786, but this feature is silly. I don't at all care that somebody that wants to cast an enum to a float must first cast to an int.

Nominating.

[emberian]

+1

[pnkfelix]

Assigning P-backcompat-lang, 1.0.

[alexcrichton]

Note that casting to other scalars, like pointers, currently ICEs

enum A {
    B,
    C,
}

fn main() {
    B as *int;
}

error: internal compiler error: translating unsupported cast: A (cast_enum) -> *int (cast_pointer)
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'Box<Any>', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libsyntax/diagnostic.rs:162