Stop setting re2_INCLUDE_DIR.

It was probably neither needed nor used and will start causing
problems as of CMake 3.29, which removes `PACKAGE_PREFIX_DIR`.

Fixes #488.

Change-Id: I707dc903234309698a6745d2b61279fd3b7bd6dc
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62930
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

re2Config.cmake.in

@@ -6,8 +6,6 @@
 
 include(CMakeFindDependencyMacro)
 
-set_and_check(re2_INCLUDE_DIR ${PACKAGE_PREFIX_DIR}/@CMAKE_INSTALL_INCLUDEDIR@)
-
 if(UNIX)
   set(THREADS_PREFER_PTHREAD_FLAG ON)
   find_dependency(Threads REQUIRED)