After some consideration, revert last commit and don't abuse rde_dump_done.

After some consideration, revert last commit and don't abuse rde_dump_done.
IMSG_CTL_SHOW_RIB_PREFIX case is not using rib_dump_r and so should not use
rde_dump_done, instead send the IMSG_CTL_END msg and free the ctx directly.
This is easier to understand.