Support only PGXS

Nobody's gonna compile it in the contrib modules anyway.

Makefile

@@ -1,15 +1,8 @@
-# contrib/pg_log_userqueries/Makefile
+# pg_log_userqueries/Makefile
 
 MODULE_big = pg_log_userqueries
 OBJS = pg_log_userqueries.o
 
-ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)
 include $(PGXS)
-else
-subdir = contrib/pg_log_userqueries
-top_builddir = ../..
-include $(top_builddir)/src/Makefile.global
-include $(top_srcdir)/contrib/contrib-global.mk
-endif