Simplify local drv goal a bit more

- `chrootParentDir` can be a local variable instead of a class variable.

- `getChildStatus` can be inlined. Again, we have the `assert(!hook);`
  in the local building case, which makes for a simpler thing inlined.

src/libstore/unix/build/local-derivation-goal.cc

@@ -282,7 +282,7 @@ Goal::Co LocalDerivationGoal::tryLocalBuild()
        to have terminated.  In fact, the builder could also have
        simply have closed its end of the pipe, so just to be sure,
        kill it. */
-    int status = getChildStatus();
+    int status = pid.kill();
 
     debug("builder process for '%s' finished", worker.store.printStorePath(drvPath));
 
@@ -412,12 +412,6 @@ static void movePath(const Path & src, const Path & dst)
 extern void replaceValidPath(const Path & storePath, const Path & tmpPath);
 
 
-int LocalDerivationGoal::getChildStatus()
-{
-    return hook ? DerivationGoal::getChildStatus() : pid.kill();
-}
-
-
 bool LocalDerivationGoal::cleanupDecideWhetherDiskFull()
 {
     bool diskFull = false;
@@ -787,7 +781,7 @@ void LocalDerivationGoal::startBuilder()
            environment using bind-mounts.  We put it in the Nix store
            so that the build outputs can be moved efficiently from the
            chroot to their final location. */
-        chrootParentDir = worker.store.Store::toRealPath(drvPath) + ".chroot";
+        auto chrootParentDir = worker.store.Store::toRealPath(drvPath) + ".chroot";
         deletePath(chrootParentDir);
 
         /* Clean up the chroot directory automatically. */

src/libstore/unix/build/local-derivation-goal.hh

@@ -71,13 +71,6 @@ struct LocalDerivationGoal : public DerivationGoal
      */
     bool useChroot = false;
 
-    /**
-     * The parent directory of `chrootRootDir`. It has permission 700
-     * and is owned by root to ensure other users cannot mess with
-     * `chrootRootDir`.
-     */
-    Path chrootParentDir;
-
     /**
      * The root of the chroot environment.
      */
@@ -237,8 +230,6 @@ struct LocalDerivationGoal : public DerivationGoal
      */
     void chownToBuilder(const Path & path);
 
-    int getChildStatus() override;
-
     /**
      * Run the builder's process.
      */