Remove linking RMM library. (#6146)

* Remove linking RMM library.

* RMM is now header only.

* Remove remaining reference.

plugin/CMakeLists.txt

@@ -13,27 +13,21 @@ if (PLUGIN_DENSE_PARSER)
 endif (PLUGIN_DENSE_PARSER)
 
 if (PLUGIN_RMM)
-  find_path(RMM_INCLUDE "rmm"
-    HINTS "$ENV{RMM_ROOT}/include")
-
-  find_library(RMM_LIBRARY "rmm"
-    HINTS "$ENV{RMM_ROOT}/lib" "$ENV{RMM_ROOT}/build")
-
-  if ((NOT RMM_LIBRARY) OR (NOT RMM_INCLUDE))
+  find_path(RMM_INCLUDE "rmm" HINTS "$ENV{RMM_ROOT}/include")
+  if (NOT RMM_INCLUDE)
     message(FATAL_ERROR "Could not locate RMM library")
   endif ()
 
   message(STATUS "RMM: RMM_LIBRARY set to ${RMM_LIBRARY}")
   message(STATUS "RMM: RMM_INCLUDE set to ${RMM_INCLUDE}")
 
   target_include_directories(objxgboost PUBLIC ${RMM_INCLUDE})
-  target_link_libraries(objxgboost PUBLIC ${RMM_LIBRARY} cuda)
   target_compile_definitions(objxgboost PUBLIC -DXGBOOST_USE_RMM=1)
 endif (PLUGIN_RMM)
 
 if (PLUGIN_UPDATER_ONEAPI)
   add_library(oneapi_plugin OBJECT
-    ${xgboost_SOURCE_DIR}/plugin/updater_oneapi/regression_obj_oneapi.cc 
+    ${xgboost_SOURCE_DIR}/plugin/updater_oneapi/regression_obj_oneapi.cc
     ${xgboost_SOURCE_DIR}/plugin/updater_oneapi/predictor_oneapi.cc)
   target_include_directories(oneapi_plugin
     PRIVATE