Remove unnecessary boost signals find_package

With Boost >1.69 hector_mapping won't build. Furthermore, hector_mapping doesn't use signals anywhere.

hector_mapping/CMakeLists.txt

@@ -7,7 +7,7 @@ project(hector_mapping)
 find_package(catkin REQUIRED COMPONENTS roscpp nav_msgs visualization_msgs tf message_filters laser_geometry tf_conversions message_generation)
 
 ## System dependencies are found with CMake's conventions
-find_package(Boost REQUIRED COMPONENTS thread signals)
+find_package(Boost REQUIRED COMPONENTS thread)
 include_directories(${Boost_INCLUDE_DIRS})
 
 # Find Eigen3 (from http://wiki.ros.org/jade/Migration)