smgrdounlinkfork() became dead code as the result of commit ece01aae,
but it was left in place just in case we want it someday. However no users
have appeared in 7 years, so it's time to remove this unused function.

Author: Kirk Jamison
Discussion: https://www.postgresql.org/message-id/D09B13F772D2274BB348A310EE3027C64E2067@g01jpexmbkw24