Dangling `dm-launchpad` references

[hartikainen]

b9522b0 removed the dependency from dm-launchpad. There's still a couple of dangling dm-launchpad references left, however, e.g. in

xmanager/docs/parameter_controller.md

Line 97 in 7a9cfd0

dm-launchpad[tensorflow]

and

xmanager/examples/parameter_controller/requirements.txt

Line 4 in 7a9cfd0

dm-launchpad[tensorflow]