Rename panda_arm.xacro -> arm.xacro

to be more consistent with hand.xacro, which doesn't have the panda prefix either.

config/panda.srdf.xacro

@@ -4,7 +4,7 @@
     A URDF file must exist for this robot as well, where the joints and the links that are referenced are defined
 -->
 <robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="panda">
-    <xacro:include filename="$(find panda_moveit_config)/config/panda_arm.xacro" />
+    <xacro:include filename="$(find panda_moveit_config)/config/arm.xacro" />
     <xacro:arm />
 
     <xacro:arg name="hand" default="false" />