rtr_mgr: remove array bound check in rtr_mgr_status_to_str
rtr_mgr: remove array bound check in rtr_mgr_status_to_str

Trust the user that he uses the function correctly and only pass valid
rtr_mgr_status values instead of random integers.

The previous array bound check also was wrong. The sizeof operations
returned the total bytes allocated of the array and not the members of
the array.

rtrlib/rtrlib,rtrlib/rtrlib,rtrlib/rtrlib