Remove the default cache_dir location and the null store type

Remove the default cache_dir location and the null store type

Many people gets confused by the builtin cache_dir location, thinking
that if there is no cache_dir in squid.conf then there is no on-disk cache.
This removes the builtin default.

By removing the builtin default we can also remove the "null" cache_dir
type whos purpose is only to override the builtin default.