Get rid of MemberImpl::Derived

We don't need this anymore, since derived members become Impls during
canonicalization now!

crates/compiler/load_internal/src/file.rs

@@ -4464,7 +4464,7 @@ fn run_solve_solve(
                 .values()
                 .any(|resolved_impl| match resolved_impl {
                     ResolvedImpl::Impl(specialization) => specialization.symbol == sym,
-                    ResolvedImpl::Derived | ResolvedImpl::Error => false,
+                    ResolvedImpl::Error => false,
                 })
         };