Remove PARTIAL_LINKING build mode.

In 9956ddc1, ten years ago, the
current objfile.txt based linking model was introduced.  It's time to
retire the old SUBSYS.o based model.

This primarily is pertinent because the bitcode files for LLVM based
inlining are not produced when using PARTIAL_LINKING. It does not seem
worth to fix PARTIAL_LINKING to support that.

Author: Andres Freund
Discussion: https://postgr.es/m/20180121204356.d5oeu34jetqhmdv2@alap3.anarazel.de

src/backend/common.mk

@@ -8,13 +8,7 @@
 # this directory and SUBDIRS to subdirectories containing more things
 # to build.
 
-ifdef PARTIAL_LINKING
-# old style: linking using SUBSYS.o
-subsysfilename = SUBSYS.o
-else
-# new style: linking all object files at once
 subsysfilename = objfiles.txt
-endif
 
 SUBDIROBJS = $(SUBDIRS:%=%/$(subsysfilename))