Remove signals from find_package(Boost COMPONENTS ...).

tf2 is using signals2, which is not the same library.
Additionally, signals2 has always been header only, and header only
libraries must not be listed in find_package.

Boost 1.69 removed the old signals library entirely, so the otherwise
useless `COMPONENTS signals` actually breaks the build.

tf2/CMakeLists.txt

@@ -3,7 +3,7 @@ project(tf2)
 
 find_package(console_bridge REQUIRED)
 find_package(catkin REQUIRED COMPONENTS geometry_msgs rostime tf2_msgs)
-find_package(Boost REQUIRED COMPONENTS signals system thread)
+find_package(Boost REQUIRED COMPONENTS system thread)
 
 catkin_package(
    INCLUDE_DIRS include