pkg-config: Drop broken include path

The legacy `includedir` was removed in 4.0 (https://github.com/opencv/opencv/pull/12477) but it was not removed from the `.pc` file.

This causes issues when trying to link against OpenCV using pkg-config in CMake.

cmake/templates/opencv-XXX.pc.in

@@ -3,12 +3,11 @@
 prefix=@prefix@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
-includedir_old=@includedir@/opencv
-includedir_new=@includedir@
+includedir=@includedir@
 
 Name: OpenCV
 Description: Open Source Computer Vision Library
 Version: @OPENCV_VERSION_PLAIN@
 Libs: @OPENCV_PC_LIBS@
 Libs.private: @OPENCV_PC_LIBS_PRIVATE@
-Cflags: -I${includedir_old} -I${includedir_new}
+Cflags: -I${includedir}