remove defaulting to unit

Types will no longer default to `()`, instead always defaulting to `!`.
This disables the associated warning and removes the flag from TyTuple

src/librustc_lint/lib.rs

@@ -234,11 +234,6 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             reference: "issue #39207 <https://github.com/rust-lang/rust/issues/39207>",
             epoch: None,
         },
-        FutureIncompatibleInfo {
-            id: LintId::of(RESOLVE_TRAIT_ON_DEFAULTED_UNIT),
-            reference: "issue #39216 <https://github.com/rust-lang/rust/issues/39216>",
-            epoch: None,
-        },
         FutureIncompatibleInfo {
             id: LintId::of(MISSING_FRAGMENT_SPECIFIER),
             reference: "issue #40107 <https://github.com/rust-lang/rust/issues/40107>",