pgsql: Remove smgrdounlink() in smgr.c from the code tree

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e111c9f90ab6090859127cbb22156f4858b6a40c

Modified Files
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src/backend/storage/smgr/smgr.c | 56 -----------------------------------------
src/include/storage/smgr.h      |  1 -
2 files changed, 57 deletions(-)