derivation-goal.cc: remove bmCheck custom return branch on buildDone

Once a derivation goal has been completed, we check whether or not
this goal was meant to be repeated to check its output.

An early return branch was preventing the worker to reach that repeat
code branch, hence breaking the --check command (#2619).

It seems like this early return branch is an artifact of a passed
refactoring. As far as I can tell, buildDone's main branch also
cleanup the tmp directory before returning.

src/libstore/build/derivation-goal.cc

@@ -914,12 +914,6 @@ void DerivationGoal::buildDone()
             outputPaths
         );
 
-        if (buildMode == bmCheck) {
-            cleanupPostOutputsRegisteredModeCheck();
-            done(BuildResult::Built, std::move(builtOutputs));
-            return;
-        }
-
         cleanupPostOutputsRegisteredModeNonCheck();
 
         /* Repeat the build if necessary. */