Remove unbounded cache

[jonnystoten]

TUFMetaStorage uses an in-memory cache to store all retrieved metadata files. This cache has a few problems:

1. It doesn't have any sort of limit on its size so it will grow indefinitely as more content is retrieved
2. Entries in the cache are not deleted when Delete is calls on a TUFMetaStorage instance. This means the cache can hold entries that have been deleted from the underlying storage (there seem to be tests for this behavior so could this be for a reason?)
3. It isn't safe for concurrent use, see Fix a concurrency bug in TUFMetaStorage by using sync.Map #1613.

I've taken the scorched-earth approach of simply removing this cache. After #1639 it should be fast to get these files from the database each time. I'll test this on Docker's production instance before merging it to check the performance is still OK.

[jonnystoten]

Looks good on Docker's notary server after a few days. The performance seems fine and more consistent - the average is a little bit slower (~3ms -> ~5ms) but the worst case is much better (>100ms -> ~8ms).

I'm assuming the tests that were testing that deleting from the TUF store doesn't delete from the cache were just wrong - anything relying on this will have other issues because of course if the server is restarted the cache will be cleared and if more than one server is running each one will have different cache contents etc.

This can also close #1613.