pgsql: Stop using "-multiply_defined suppress" on macOS.

Stop using "-multiply_defined suppress" on macOS.

We started to use this linker switch in commit 9df308697 of
2004-07-13, which was in the OS X 10.3 era.  Apparently it's been a
no-op since around OS X 10.9.  Apple's most recent toolchain version
actively complains about it, so it's time to get rid of it.

Discussion: https://postgr.es/m/467042.1695766998@sss.pgh.pa.us

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/abe423712a12bfd53d2e35b5ca06f599ed926b9a

Modified Files
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src/Makefile.shlib | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)