slapos-master: delete custom instance-erp5.cfg.in

Historically, a custom instance-erp5.cfg.in was required for SlapOS Master in order to install the SlapOS configurator. This required changing the default business templates, which can, since 49a7f256, be done using a custom build section.

This commit updates the software.cfg file accordingly, and deletes the legacy instance-erp5.cfg.in file, which is not needed anymore, to instead use the file provided in stack/erp5.

/cc @rafael