Trying to push the condition to the database query will result in poor performance. Instead, only add the condition if it is required:
var deleteItems = _EFContext.DbTable.Where(so => so.Identifier == ProviderIdentifier);
if (!string.IsNullOrEmpty(tranType))
{
deleteItems = deleteItems.Where(so => so.TranType == tranType);
}
_EFContext.DbTable.RemoveRange(deleteItems);
_EFContext.SaveChanges();