Avoid clearing HaarEvaluator::tofs variable (introduced…

Avoid clearing HaarEvaluator::tofs variable (introduced fb8478eb) in HaarEvaluator::setImage(...) since it won't be recomputed by HaarEvaluator::computeOptFeatures() for cases where the input image dimensions and requested search scales are the same as the previous.  If none of the above change then it should be correct, and if they do change it will be computed correctly inside HaarEvaluator::computeOptFeatures() (in commit 952f9dbe)

I'm guessing there is a unit test for this that needs to be called 2x with the same image and search parameters (or another image with the same size) to illustrate the problem.

modules/objdetect/src/cascadedetect.cpp

@@ -680,7 +680,6 @@ void HaarEvaluator::computeOptFeatures()
 }
 
 bool HaarEvaluator::setImage(InputArray _image, const std::vector<float>& _scales){
-    tofs = 0;
     return FeatureEvaluator::setImage(_image, _scales);
 }