software/helloworld: Switch to default go version

Patch nexedi/slapos@15e26abd explicitly declared
go version 1.17 for the 'helloworld' SR.

This was done at a time when we already had [golang1.17] in component/golang,
but the default was still go1.16. The explicit declaration allowed the 'helloworld' SR
to already switch to the newest go version.

Since patch nexedi/slapos@d32524c3
we use go1.18 as our default version. In order to allow 'helloworld' to use our newest
SlapOS supported go version, it's therefore sufficient nowadays to simply remove the
explicit declaration.

/reviewed-on nexedi/slapos!1330
/reviewed-by @kirr