cmd/cue: remove skipped test scripts for fixing old definitions

These tests have been disabled since `cue fix` stopped fixing old-style
definitions like `foo :: bar`, back in July 2020
via https://cue-review.googlesource.com/c/cue/+/6656.

That change disabled the scripts rather than removing them entirely,
with a note to keep them around until we were sure about the change.

It's been nearly three years since the tests were disabled,
and support for `::` was removed entirely in February 2023 via
https://cuelang.org/cl/546886, so there's no going back for sure.

The test scripts are always in the git history for reference if needed.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I2a90f69cbf9c90c99aef851781e19730f4b08c47
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/552384
Unity-Result: CUEcueckoo <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>