Remove the unknown insert validation.

There's actually a legitimate case where this is ok: right after a
backup restore.

Restoring a backup means that you have possibly carried over some
unknownIds, and if you don't remember your PIN, those items wouldn't be
there remotely. And you _should_ insert them. Otherwise they're lost.

I don't think this validation is worth the trouble of carving out lots
of conditions to allow this usecase.

app/src/main/java/org/thoughtcrime/securesms/storage/StorageSyncValidations.java

@@ -137,10 +137,6 @@ private static void validateManifestAndInserts(@NonNull SignalStorageManifest ma
         throw new InsertNotPresentInFullIdSetError();
       }
 
-      if (insert.isUnknown()) {
-        throw new UnknownInsertError();
-      }
-
       if (insert.getContact().isPresent()) {
         SignalContactRecord contact = insert.getContact().get();
 
@@ -173,9 +169,6 @@ private static final class InsertNotPresentInFullIdSetError extends Error {
   private static final class DeletePresentInFullIdSetError extends Error {
   }
 
-  private static final class UnknownInsertError extends Error {
-  }
-
   private static final class MultipleAccountError extends Error {
   }