Revert "cmd/cue: add test for the loss of comments with cue def"

This reverts commit 43a16ed.

Reason for revert: we attempted to rebase the author's patch on Gerrit
but instead of rebasing the latest PS8, we rebased PS3,
meaning all the latest changes were not merged correctly.
We are reverting the older PS3 changes to then merge PS8 cleanly.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I357123032c55b9f88e4565c1287c947304ece136
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1204838
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>