Clean some functions about project issue (#27705)

1. remove unused function `MoveIssueAcrossProjectBoards`
2. extract the project board condition into a function
3. use db.NoCondition instead of -1. (BTW, the usage of db.NoCondition
is too confusing. Is there any way to avoid that?)
4. remove the unnecessary comment since the ctx refactor is completed.
5. Change `b.ID != 0` to `b.ID > 0`. It's more intuitive but I think
they're the same since board ID can't be negative.

modules/indexer/issues/db/options.go

@@ -96,7 +96,6 @@ func ToDBOptions(ctx context.Context, options *internal.SearchOptions) (*issue_m
 		}
 
 		if len(options.IncludedLabelIDs) == 0 && len(options.IncludedAnyLabelIDs) > 0 {
-			_ = ctx // issue_model.GetLabelsByIDs should be called with ctx, this line can be removed when it's done.
 			labels, err := issue_model.GetLabelsByIDs(ctx, options.IncludedAnyLabelIDs, "name")
 			if err != nil {
 				return nil, fmt.Errorf("GetLabelsByIDs: %v", err)