3.4: -Wunused-but-set-variable warnings fix after XCode update

[asmorkalov]

Patch for 4.x: #3434
Merge with opencv/opencv#23214

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[alalek]

Where are GitHub actions results?

[alalek]

One more warning left (from main PR): https://github.com/opencv/opencv/actions/runs/4101618585/jobs/7075813139