Move derivationType from DerivationGoal to LocalDerivationGoal

The super class doesn't actually care.

src/libstore/build/derivation-goal.cc

@@ -533,9 +533,6 @@ Goal::Co DerivationGoal::gaveUpOnSubstitution()
 
     debug("added input paths %s", worker.store.showPaths(inputPaths));
 
-    /* What type of derivation are we building? */
-    derivationType = drv->type();
-
     /* Okay, try to build.  Note that here we don't wait for a build
        slot to become available, since we don't need one if there is a
        build hook. */

src/libstore/build/derivation-goal.hh

@@ -198,11 +198,6 @@ struct DerivationGoal : public Goal
     std::unique_ptr<HookInstance> hook;
 #endif
 
-    /**
-     * The sort of derivation we are building.
-     */
-    std::optional<DerivationType> derivationType;
-
     BuildMode buildMode;
 
     std::unique_ptr<MaintainCount<uint64_t>> mcExpectedBuilds, mcRunningBuilds;