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