Revert "If a substitute closure is incomplete, build dependencies, then retry the substituter"

As summarized in
https://github.com/NixOS/nix/issues/77#issuecomment-2843228280 the
motivation is that the complicated retry logic this introduced was
making the cleanup task #12628 harder to accomplish. It was not easy to
ascertain just what policy / semantics the extra control-flow was
implementing, in order to figure out a different way to implementing it
either.

After talking to Eelco about it, he decided we could just....get rid of
the feature entirely! It's a bit scary removing a decade+ old feature,
but I think he is right. See the release notes for more explanation.

This reverts commit 299141ecbd.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libstore/build/substitution-goal.cc

@@ -169,7 +169,7 @@ Goal::Co PathSubstitutionGoal::tryToRun(StorePath subPath, nix::ref<Store> sub,
 
     if (nrFailed > 0) {
         co_return done(
-            nrNoSubstituters > 0 || nrIncompleteClosure > 0 ? ecIncompleteClosure : ecFailed,
+            nrNoSubstituters > 0 ? ecNoSubstituters : ecFailed,
             BuildResult::DependencyFailed,
             fmt("some references of path '%s' could not be realised", worker.store.printStorePath(storePath)));
     }