[C++14] remove explicit -std=c++11 and checks

From REP-0003

> Melodic
> As of ROS Melodic, we are using the C++14 (ISO/IEC 14882:2014) standard.

Related to discussion on: moveit/moveit#1146
Similar to: ros/class_loader#87

pluginlib/CMakeLists.txt

@@ -23,15 +23,10 @@ if(CATKIN_ENABLE_TESTING)
     add_dependencies(${PROJECT_NAME}_utest test_plugins)
   endif()
 
-  include(CheckCXXCompilerFlag)
-  check_cxx_compiler_flag("-std=c++11" COMPILER_SUPPORTS_CXX11)
-  if(COMPILER_SUPPORTS_CXX11)
-    catkin_add_gtest(${PROJECT_NAME}_unique_ptr_test test/unique_ptr_test.cpp)
-    if(TARGET ${PROJECT_NAME}_unique_ptr_test)
-      target_link_libraries(${PROJECT_NAME}_unique_ptr_test ${TinyXML2_LIBRARIES} ${catkin_LIBRARIES} ${Boost_LIBRARIES})
-      set_target_properties(${PROJECT_NAME}_unique_ptr_test PROPERTIES COMPILE_FLAGS -std=c++11 LINK_FLAGS -std=c++11)
-      add_dependencies(${PROJECT_NAME}_unique_ptr_test test_plugins)
-    endif()
+  catkin_add_gtest(${PROJECT_NAME}_unique_ptr_test test/unique_ptr_test.cpp)
+  if(TARGET ${PROJECT_NAME}_unique_ptr_test)
+    target_link_libraries(${PROJECT_NAME}_unique_ptr_test ${TinyXML2_LIBRARIES} ${catkin_LIBRARIES} ${Boost_LIBRARIES})
+    add_dependencies(${PROJECT_NAME}_unique_ptr_test test_plugins)
   endif()
 
 endif()