Remove unused ttl field from cache.Cache. (#2658)

The cache itself isn't responsible for snapshot invalidation anymore,
but this field has been left behind.

Signed-off-by: Ben Luddy <bluddy@redhat.com>

pkg/controller/registry/resolver/cache/cache.go

@@ -86,7 +86,6 @@ type Cache struct {
  sp SourceProvider
  sourcePriorityProvider SourcePriorityProvider
  snapshots map[SourceKey]*snapshotHeader
- ttl time.Duration
  sem chan struct{}
  m sync.RWMutex
 }
@@ -121,7 +120,6 @@ func New(sp SourceProvider, options ...Option) *Cache {
  sp: sp,
  sourcePriorityProvider: constantSourcePriorityProvider(0),
  snapshots: make(map[SourceKey]*snapshotHeader),
- ttl: 5 * time.Minute,
  sem: make(chan struct{}, MaxConcurrentSnapshotUpdates),
  }