Drop unneeded dependency on python-enum34 (#2819)

The enum module was added in PEP 435 and is present in Python versions
3.4 and newer, which includes all currently supported Python versions.

nav2_simple_commander/package.xml

@@ -7,7 +7,6 @@
   <maintainer email="stevenmacenski@gmail.com">steve</maintainer>
   <license>Apache-2.0</license>
 
-  <exec_depend>python-enum34</exec_depend>
   <exec_depend>rclpy</exec_depend>
   <exec_depend>geometry_msgs</exec_depend>
   <exec_depend>nav2_msgs</exec_depend>