slapos-testing, slapos-sr-testing: Don't duplicate instance.cfg prologue that is already provided by stack/nxdtest/instance.cfg

This software releases switched to nxdtest in 1536ad2e (software/slapos-testing: use nxdtest)
and 3a1e5628 (software/slapos-sr-testing: use nxdtest). So there is no
longer need to duplicate that standard instance prologue that is
provided by stack/nxdtest/instance.cfg:

https://lab.nexedi.com/nexedi/slapos/blob/0803b816/stack/nxdtest/instance.cfg.in#L1-8

/reviewed-by @jerome
/reviewed-on !94