Revert "processes_linux_p: ensure to have a single smaps read runnable"

This reverts commit 1f48d7e9

This commit causes VmPSS to only be updated for one entry every update cycle, which means many processes don't show the proper memory usage.

This is due to storing a single runnable in the private object but we need a runnable for each PID that should be checked.

@meven