Remove civicrm_entity_financial_item table and always use civicrm_batch_entity to link financial_trxn rows to batches

Currently civicrm_entity_financial_item is being used to link financial_item rows to batches (when assigning transactions to financial batches). This is incorrect - that flow should be assigning finanial_trxn rows to batches via civicrm_entity_batch.

The civicrm_entity_financial_item table should be removed from the schema and references in code as well as upgrades should be checked / fixed or removed. For example:

CRM_Financial_BAO_EntityFinancialItem::getBatchFinancialItems references that table. That method is called in:
CRM/Financial/Form/BatchTransaction.php:107
CRM/Financial/Page/BatchTransaction.php:139