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