[#1700] engine: Drop unused block execution methods

`BlockExecution` and `ResumeExecution` were used only by unit test.
So drop them and simplify code.

Change-Id: Ib3de324617e8a27fc1f015542ac5e94df5c60a6e
Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/engine/engine.go

@@ -33,9 +33,8 @@ type StorageEngine struct {
 	wg        sync.WaitGroup
 
 	blockExec struct {
-		mtx sync.RWMutex
-
-		err error
+		mtx    sync.RWMutex
+		closed bool
 	}
 	evacuateLimiter *evacuationLimiter
 }