Remove what was only there to move a commit

A line in postgres's recipe and a one-line file in amqp-ping, both added to
move a commit and watch the mesh notice. The proofs worked; neither was meant
to stay. MESH_MODULE is set from the sealed credential at run time anyway, so
baking it in was dead weight as well as noise.

modules/amqp-ping/README.md

@@ -1 +0,0 @@
-// touched to move the commit: proving an upgrade is announced

modules/postgres/Dockerfile

@@ -34,5 +34,3 @@ COPY --from=build /app/modules/postgres/dist /app/modules/postgres/dist
 # and is not listed here — the declaration names it in the container's `args`, because it is what
 # this module's own container runs. One image, because they are one module and share a client.
 ENV MESH_TOOL_MODULES=/app/modules/postgres/dist/index.js,/app/modules/postgres/dist/tools/index.js
-# A real change, so the artifact itself moves and not only the commit.
-ENV MESH_MODULE=postgres