Deprecate/remove unused checkVersion() functions

[colemanw]

Overview

Deprecate 1 unused function and remove another unused function.

Before

2 unused functions.

After

CRM_Core_DAO::checkVersion() marked as deprecated.
CRM_Upgrade_Form::checkVersion() removed.

Comments

Both functions are unused according to my grepping. The one in CRM_Core_DAO could theoretically be used somewhere in the extension universe (though I doubt it), so I marked it as deprecated to provide a transition period.

The other function in CRM_Upgrade_Form is very "internal" and IMO very unlikely to be in use anywhere, so I deleted it.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[demeritcowboy]

I don't see where they are used in core either. And the upgrade one looks like it was used in earlier versions of incremental upgrades: civicrm/civicrm-svn@dc511ab#diff-49f0a52691f7a2889a9f20651f317956