Fix for following build_js warnings

/build/precommit_docs/4.x/opencv/modules/imgproc/src/color_lab.cpp:965:19: warning: unused variable 'enablePackedLab' [-Wunused-const-variable]
/build/precommit_docs/4.x/opencv/modules/imgproc/src/color_lab.cpp:982:19: warning: unused variable 'enablePackedRGB2Luv' [-Wunused-const-variable]
/build/precommit_docs/4.x/opencv/modules/imgproc/src/color_lab.cpp:983:19: warning: unused variable 'enablePackedLuv2RGB' [-Wunused-const-variable]
/build/precommit_docs/4.x/opencv/modules/imgproc/src/color_lab.cpp:1387:20: warning: unused function 'trilinearPackedInterpolate' [-Wunused-function]

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