Automatic merge from submit-queue (batch tested with PRs 48405, 48742, 48748, 48571, 48482)

dockershim: clean up unused security context code

Most of the code in the `dockershim/securitycontext` package are
unused and can be removed. This PR migrates the rest of the code,
cleans it up (e.g., remove references to kubernetes API objects),
and removes the package entirely.