-Wunused-but-set-variable warnings fix after XCode update with clang 13.1

[asmorkalov]

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]

One more fix is required:

/Users/opencv-cn/GHA-OCV-1/_work/opencv_contrib/opencv_contrib/opencv_contrib/modules/ximgproc/src/brightedges.cpp:82:17: warning: variable 'lines' set but not used [-Wunused-but-set-variable]

[asmorkalov]

@alalek The last warning is tricky. The issue looks like typo in code. Sample result before and after fix: