Setuid and rpath were bad ideas here. Removing.

Summary:
 - the deb build process strips the setuid bit; it should be set elsewhere
    for debs if it's necessary.
 - pr2-grant is still required for anyone building pr2_robot from source
 - adding rpath here is insufficient; it's also required for all of the plugins
    that get loaded by pr2_ethercat at runtime. We should pass LD_LIBRARY_PATH
    into pr2_ethercat instead.