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