Remove nti.zodb.common

[jamadden]

Its function make_cache_dir seems to have been copied from somewhere else and doesn't belong here.

It's used by ZlibStorageClientStorageURIFactory to set a location for a cache file, but I think we can just use the default if the user doesn't specify a var directory.

[papachoco]

I remembered I copied it from somewhere to make the nti.zodb independent