Retire also the reference to VSM_NOPID in docs

Ref previous commit to remove the use of kill(signull) on the pids of the
Varnish processes to test liveness.

doc/sphinx/reference/vsm.rst

@@ -97,22 +97,3 @@ a chance to discover the deallocation.
 
 The include file <vapi/vsm.h> provides the supported API for accessing
 VSM files.
-
-VSM and Containers
-------------------
-
-The varnish way works great with single purpose containers. By sharing
-the varnish working directory read-only, vsm readers can be run in
-containers separate from those running varnishd instances on the same
-host.
-
-When running varnishd and vsm readers in the same process namespace,
-pid information can be used by vsm readers to determine if varnishd
-processes are alive.
-
-But, when running varnishd and vsm readers in different containers,
-the pid information has no relevance and may even be ambiguous across
-name spaces.
-
-Thus, with such setups, the environment variable VSM_NOPID needs to be
-set for vsm readers to disable use of pid information.