remove unneeded dockerignore file

It was added as part of https://cue-review.googlesource.com/c/cue/+/5140
as that initial Dockerfile built cmd/cue inside a docker container,
so the dockerignore file helped the COPY line to not copy too much.
The go build has happened outside of Docker for a long time now,
and the Dockerfile only does a COPY of a single binary.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I9b7e686b34b06b12b29e591359c1b0ba9118e13c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1183761
TryBot-Result: CUEcueckoo <cueckoo@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.io>