core: str::as_slice is unneeded, yay! fixes std::net::tcp socket_buf test

i mistook an "unconstrained type" error, due to type-inference messup
because i didnt have return vals in some closure wired-up right, for being
due to not having a str as a str/& (a str will actually auto-coerce to a
str/&, so str::as_slice was erroneously added. my bad).

src/libcore/str.rs

@@ -23,7 +23,6 @@ export
    as_buf,
    as_c_str,
    unpack_slice,
-   as_slice,
 
    // Adding things to and removing things from a string
    push_char,
@@ -1621,16 +1620,6 @@ pure fn unpack_slice<T>(s: str/&, f: fn(*u8, uint) -> T) -> T {
     }
 }
 
-pure fn as_slice<T>(s: str,
-                    start: uint, end: uint,
-                    f: fn(str/&) -> T) -> T unsafe {
-    assert (start <= end);
-    assert (end <= len(s));
-    let p = ptr::addr_of(s);
-    f(::unsafe::reinterpret_cast(
-        (ptr::offset(p, start), (end - start) * sys::size_of::<u8>())))
-}
-
 #[doc = "
 Reserves capacity for exactly `n` bytes in the given string, not including
 the null terminator.