cmd/cue: remove go install long tests

These testscripts verify that the `go install` instructions in
docs like the README work when using proxy.golang.org.

They are quite expensive; even with a warm cache, they take multiple
seconds to run, as they do network roundtrips and re-link binaries.
That is why they have always been long tests.

However, they aren't good tests to keep going forward.
We want to start having CI always run all long tests,
because otherwise it's far too easy to forget them while testing CLs.

Moreover, we will start writing interactive guides in the form of
testable scripts which would run these same `go install` commands,
so those would already serve the purpose of verifying that installing
CUE works in a fully end-to-end way.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I9b69ed7ce9295ed8a6738a88aa278b2136250ae6
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/551228
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>