internal/_e2e: remove cleanup.go and tools.go

Since https://cuelang.org/cl/1178251, we no longer create GitHub repos
for each testscript run, so we no longer need to worry about any
cleanup of repositories we left behind due to crashes or bugs.

Similarly, since https://cuelang.org/cl/1178256, the e2e tests
are no longer their own separate Go module, so we don't need a tools.go
to track cuelang.org/go/cmd/cue as a tool dependency in go.mod.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ib661db94df08cec9546e10089e1e0239df89d872
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1178270
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@gmail.com>