x/build: remove OpenBSD 6.0 builder

[bradfitz]

Per https://golang.org/doc/go1.10#ports ,

Go 1.10 is the last release that will run on OpenBSD 6.0. Go 1.11 will require OpenBSD 6.2.

Remove the two OpenBSD 6.0 builders (386 and amd64). They'll still be available via gomote for another release or two, but they shouldn't run proactively.

[gopherbot]

Change https://golang.org/cl/97417 mentions this issue: dashboard: disable OpenBSD 6.0 builders