pgsql: Remove SUBSYS.o rule in common.mk, hasn't been used in a long ti

Remove SUBSYS.o rule in common.mk, hasn't been used in a long time

Apparently I missed that this SUBSYS.o rule isn't needed anymore in
a4ebbd27527, likely because there still is a reference to it due to AIX - but
that's self contained in src/backend/Makefile

Discussion: https://postgr.es/m/20220820174213.d574qde4ptwdzoqz@awork3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/05bf551040ba161ddfbf7da308d61d2b7b425bf3

Modified Files
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src/backend/common.mk | 3 ---
1 file changed, 3 deletions(-)