FindBoost: Generate imported targets for future Boost versions

Per discussion on cmake/cmake#17575, this protection not particularly
valuable, as the dependency information which the imported targets wrap
is generated anyway.

This removes a road-block for using `Boost_ADDITIONAL_VERSIONS` to
support newly-released Boost versions pending a new CMake release.

Modules/FindBoost.cmake

@@ -793,7 +793,6 @@ function(_Boost_COMPONENT_DEPENDENCIES component _ret)
     endif()
     if(NOT Boost_VERSION VERSION_LESS 106700)
       message(WARNING "New Boost version may have incorrect or missing dependencies and imported targets")
-      set(_Boost_IMPORTED_TARGETS FALSE)
     endif()
   endif()