Remove unused smgrdounlinkfork() function.
smgrdounlinkfork() became dead code as the result of commit
ece01aae47,
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