Fix double del when setting dnn option to False

If opnecv is built with both dnn and with_cuda False, the build fails, because the conanfile.py execute 
del self.options.dnn_cuda twice. 
This fixes the issue.

recipes/opencv/4.x/conanfile.py

@@ -155,13 +155,12 @@ def configure(self):
         if not self.options.contrib:
             del self.options.contrib_freetype
             del self.options.contrib_sfm
-        if not self.options.dnn:
+        if not self.options.dnn or not self.options.with_cuda:
             del self.options.dnn_cuda
         if not self.options.with_cuda:
             del self.options.with_cublas
             del self.options.with_cudnn
             del self.options.with_cufft
-            del self.options.dnn_cuda
             del self.options.cuda_arch_bin
         if bool(self.options.with_jpeg):
             if self.options.get_safe("with_jpeg2000") == "jasper":