Revert "core.shm: $SNABB_SHM_ROOT overrides /var/run/snabb"

This had been intended as a mechanism for preserving the shm state
(including timeline) by storing it somewhere more specific than
/var/run/snabb. However, this is not effective now that Snabb
processes are automatically removing their state on termination. So a
new mechanism must be found.

This reverts commit 9746f24.

src/core/shm.lua

@@ -70,7 +70,7 @@ local S = require("syscall")
 local const = require("syscall.linux.constants")
 
 -- Root directory where the object tree is created.
-root = os.getenv("SNABB_SHM_ROOT") or "/var/run/snabb"
+root = "/var/run/snabb"
 path = ""
 
 -- Table (address->size) of all currently mapped objects.