Remove references to libpq_srcdir in adminpack and old_snapshot

Those two modules included references to libpq's source path, without
using anything from libpq.  Some copy-pastos done when each module was
created are likely at the origin of those useless references (aecf5ee2
for old_snapshot, fe59e566 for adminpack).

Reviewed-by: Tom Lane, David Rowley
Discussion: https://postgr.es/m/X+LQpfLyk7jgzUki@paquier.xyz

contrib/adminpack/Makefile

@@ -4,7 +4,6 @@ MODULE_big = adminpack
 OBJS = \
 	$(WIN32RES) \
 	adminpack.o
-PG_CPPFLAGS = -I$(libpq_srcdir)
 
 EXTENSION = adminpack
 DATA = adminpack--1.0.sql adminpack--1.0--1.1.sql adminpack--1.1--2.0.sql\

contrib/old_snapshot/Makefile

@@ -4,7 +4,6 @@ MODULE_big = old_snapshot
 OBJS = \
 	$(WIN32RES) \
 	time_mapping.o
-PG_CPPFLAGS = -I$(libpq_srcdir)
 
 EXTENSION = old_snapshot
 DATA = old_snapshot--1.0.sql