Revert Cached changes from bb02d73c4

Bring it back in line with Sonarr.  I was originally worried that we
were caching all Artists and this would be a resource hog.  But the
caching will happen on every RSS scan so I think we're better off just
holding the cache and not worrying.

src/NzbDrone.Common.Test/CacheTests/CachedFixture.cs

@@ -101,33 +101,6 @@ namespace NzbDrone.Common.Test.CacheTests
 
             hitCount.Should().BeInRange(3, 7);
         }
-
-        [Test]
-        [Retry(3)]
-        [Platform(Exclude = "MacOsX")]
-        public void should_clear_expired_when_they_expire()
-        {
-            int hitCount = 0;
-            _cachedString = new Cached<string>();
-
-            for (int i = 0; i < 10; i++)
-            {
-                _cachedString.Get("key",
-                    () =>
-                        {
-                            hitCount++;
-                            return null;
-                        },
-                    TimeSpan.FromMilliseconds(300));
-
-                Thread.Sleep(100);
-            }
-
-            Thread.Sleep(1000);
-
-            hitCount.Should().BeInRange(3, 7);
-            _cachedString.Values.Should().HaveCount(0);
-        }
     }
 
     public class Worker

src/NzbDrone.Common/Cache/Cached.cs

@@ -39,11 +39,6 @@ namespace NzbDrone.Common.Cache
         {
             Ensure.That(key, () => key).IsNotNullOrWhiteSpace();
             _store[key] = new CacheItem(value, lifetime);
-
-            if (lifetime != null)
-            {
-                System.Threading.Tasks.Task.Delay(lifetime.Value).ContinueWith(t => _store.TryRemove(key, out var temp));
-            }
         }
 
         public T Find(string key)