remove GetObject from ObjectLayer interface (#11635)

This commit removes the `GetObject` method
from the `ObjectLayer` interface.

The `GetObject` method is not longer used by
the HTTP handlers implementing the high-level
S3 semantics. Instead, they use the `GetObjectNInfo`
method which returns both, an object handle as well
as the object metadata.

Therefore, it is no longer necessary that a concrete
`ObjectLayer` implements `GetObject`.

cmd/web-handlers_test.go

@@ -789,7 +789,7 @@ func testUploadWebHandler(obj ObjectLayer, instanceType string, t TestErrHandler
 	}
 
 	var byteBuffer bytes.Buffer
-	err = obj.GetObject(context.Background(), bucketName, objectName, 0, int64(len(content)), &byteBuffer, "", ObjectOptions{})
+	err = GetObject(context.Background(), obj, bucketName, objectName, 0, int64(len(content)), &byteBuffer, "", ObjectOptions{})
 	if err != nil {
 		t.Fatalf("Failed, %v", err)
 	}