Remove contrib dependency.

The old Makefile relies heavily on been in the contrib tree. While
analysing the ethansf pull request, I realized that that premise does
not hold water because you could build it without a source code
tree.

Although I didn't have used his patch [1], this code was inspired on it.

[1] #1

Makefile

@@ -1,55 +1,18 @@
-# contrib/wal2json/Makefile
+MODULES = wal2json
 
-MODULE_big = wal2json
-OBJS = wal2json.o
+REGRESS = insert1 update1 update2 update3 update4 delete1 delete2 \
+		  delete3 delete4 savepoint specialvalue toast bytea
 
-# Note: because we don't tell the Makefile there are any regression tests,
-# we have to clean those result files explicitly
-EXTRA_CLEAN = -r $(pg_regress_clean_files)
-
-ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
-else
-subdir = contrib/wal2json
-top_builddir = ../..
-include $(top_builddir)/src/Makefile.global
-include $(top_srcdir)/contrib/contrib-global.mk
-endif
-
-# Disabled because these tests require "wal_level=logical", which
-# typical installcheck users do not have (e.g. buildfarm clients).
-installcheck:;
-
-# But it can nonetheless be very helpful to run tests on preexisting
-# installation, allow to do so, but only if requested explicitly.
-installcheck-force: regresscheck-install-force
-
-check: regresscheck
-
-submake-regress:
-	$(MAKE) -C $(top_builddir)/src/test/regress all
-
-submake-test_decoding:
-	$(MAKE) -C $(top_builddir)/contrib/test_decoding
-
-REGRESSCHECKS=insert1 update1 update2 update3 update4 delete1 delete2 delete3 delete4 \
-			  savepoint specialvalue toast bytea
-
-regresscheck: all | submake-regress submake-test_decoding
-	$(pg_regress_check) \
-		--temp-config $(top_srcdir)/contrib/test_decoding/logical.conf \
-		--temp-install=./tmp_check \
-		--extra-install=contrib/wal2json \
-		--extra-install=contrib/test_decoding \
-		$(REGRESSCHECKS)
-
-regresscheck-install-force: | submake-regress submake-test_decoding
-	$(pg_regress_installcheck) \
-		--extra-install=contrib/wal2json \
-		--extra-install=contrib/test_decoding \
-		$(REGRESSCHECKS)
 
-PHONY: check submake-regress submake-test_decoding \
-	regresscheck regresscheck-install-force
+# make installcheck
+#
+# It can be run but you need to add the following parameters to
+# postgresql.conf:
+#
+# wal_level = logical
+# max_replication_slots = 4
+#
+# Also, you should start the server before executing it.