Remove references to libpq_srcdir in adminpack and old_snapshot

 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 (aecf5ee
 for old_snapshot, fe59e56 for adminpack).
 
 Reviewed-by: Tom Lane, David Rowley
 Discussion: https://postgr.es/m/X+LQpfLyk7jgzUki@paquier.xyz