Remove Close Accounting Period code

CRM-16189: Improve support for Accrual Method bookkeeping included support for migrating data from CiviCRM to an accounting package at the summary level at the end of an accounting period, as requested numerous times over the years, rather than having all of the transaction detail go to the accounting system. When the community started pushing back at having new functionality in core, I decided this portion of the work could be relatively easily put into an extension. At another level, there was an uneasiness with putting into core some of this functionality like a trial balance and close accounting period function since this brought us too close to replicating accounting system functionality. Moving it to an extension seemed like a good idea.

A very small number of users may have made use of this unadvertised functionality, given there were lingering concerns about the trial balances calculated before the accounting periods were closed on 2nd and subsequent periods.

On upgrade, check if there are values in the fields to be dropped, and if so, install the extension, and copy the values of the fields to be dropped from the core table (civicrm_financial_account) to the extension table. If there are no values, do not install the extension and do proceed to drop the fields.