Remove unstable marker from std::rt

The `std::rt` module was marked `unstable` [a while
back](b6d4d117f4),
and this change was not reverted when we moved to an `experimental`
baseline for `std`.

src/libstd/lib.rs

@@ -263,7 +263,6 @@ pub mod fmt;
 
 // FIXME #7809: This shouldn't be pub, and it should be reexported under 'unstable'
 // but name resolution doesn't work without it being pub.
-#[unstable]
 pub mod rt;
 mod failure;