remove leftover `type_ascription` remains in feature gating

[Noratrieb]

After #109128, there's some leftover code in feature gating.

rust/compiler/rustc_ast_passes/src/feature_gate.rs

Lines 378 to 401 in 613a5c9

	fn visit_stmt(&mut self, stmt: &'a ast::Stmt) {
	if let ast::StmtKind::Semi(expr) = &stmt.kind
	&& let ast::ExprKind::Assign(lhs, _, _) = &expr.kind
	&& let ast::ExprKind::Type(..) = lhs.kind
	&& self.sess.parse_sess.span_diagnostic.err_count() == 0
	&& !self.features.type_ascription
	&& !lhs.span.allows_unstable(sym::type_ascription)
	{
	// When we encounter a statement of the form `foo: Ty = val;`, this will emit a type
	// ascription error, but the likely intention was to write a `let` statement. (#78907).
	feature_err(
	&self.sess.parse_sess,
	sym::type_ascription,
	lhs.span,
	"type ascription is experimental",
	).span_suggestion_verbose(
	lhs.span.shrink_to_lo(),
	"you might have meant to introduce a new binding",
	"let ",
	Applicability::MachineApplicable,
	).emit();
	}
	visit::walk_stmt(self, stmt);
	}

It should be removed, although it needs to be ensured that code like it does not compile afterwards (I haven't tested it).

cc @chenyukang