Remove smgrdounlink() in smgr.c from the code tree
The last caller of this routine was removed in
b416691, and as a wise
man said one day, dead code tends to silently break.
Per discussion between Fujii Masao, Peter Geoghegan, Vignesh C and me.
Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wz=sg5H8-vG4d5UmAofdcRMpeTDt2K-NUWp4GSfhenRGAQ@mail.gmail.com