Remove legacy unused IMMEDIATE CMake keyword

This was once supported in CMake 2.x and in current 3.x versions is
ignored.

Author: petk

CMakeLists.txt

@@ -489,7 +489,7 @@ include(cmake/darktable-packaging.cmake)
 configure_file(
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake/cmake_uninstall.cmake.in"
     "${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake"
-    IMMEDIATE @ONLY)
+    @ONLY)
 
 add_custom_target(uninstall
     COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake)