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