Remove xid_to_int4 postgres function

Fixes #3288

db/functions/Makefile

@@ -16,7 +16,7 @@ all: ${DESTDIR}/libpgosm.so
 clean:
 	$(RM) ${DESTDIR}/*.so ${DESTDIR}/*.o
 
-${DESTDIR}/libpgosm.so: ${DESTDIR}/quadtile.o ${DESTDIR}/xid_to_int4.o
+${DESTDIR}/libpgosm.so: ${DESTDIR}/quadtile.o
 	cc ${LDFLAGS} -o $@ $^
 
 ${DESTDIR}/%.o: %.c

db/functions/functions.sql

@@ -40,34 +40,3 @@ BEGIN
   RETURN (x << 1) | y;
 END;
 $$ LANGUAGE plpgsql IMMUTABLE;
-
-
--- xid_to_int4 converts a PostgreSQL transaction ID (xid) to a 32-bit integer
--- which can then be used to efficiently find rows which have changed between
--- two given transactions. This is currently used by Osmosis to extract a
--- stream of edits for "diff replication" **HOWEVER** this is a pain point, as
--- (ab)using the xid in this way is _not_ supported or recommended by Postgres
--- devs. It is preventing us upgrading to PostgreSQL version 10+, and will
--- hopefully be replaced Real Soon Now.
---
--- From the Osmosis distribution by Brett Henderson:
--- https://github.com/openstreetmap/osmosis/blob/master/package/script/contrib/apidb_0.6_osmosis_xid_indexing.sql
-CREATE OR REPLACE FUNCTION xid_to_int4(t xid)
-  RETURNS integer
-  AS
-$$
-DECLARE
-  tl bigint;
-  ti int;
-BEGIN
-  tl := t;
-
-  IF tl >= 2147483648 THEN
-    tl := tl - 4294967296;
-  END IF;
-
-  ti := tl;
-
-  RETURN ti;
-END;
-$$ LANGUAGE 'plpgsql' IMMUTABLE STRICT;

db/functions/xid_to_int4.c

@@ -1,22 +0,0 @@
-#include <postgres.h>
-#include <fmgr.h>
-
-Datum
-xid_to_int4(PG_FUNCTION_ARGS)
-{
-   TransactionId xid = PG_GETARG_INT32(0);
-
-   PG_RETURN_INT32(xid);
-}
-
-PG_FUNCTION_INFO_V1(xid_to_int4);
-
-/*
- * To bind this into PGSQL, try something like:
- *
- * CREATE FUNCTION xid_to_int4(xid) RETURNS int4
- *  AS '/path/to/rails-port/db/functions/libpgosm', 'xid_to_int4'
- *  LANGUAGE C IMMUTABLE STRICT;
- *
- * (without all the *s)
- */