MAINTAINERS.md: remove myself from promtool maintainers
Unfortunately I don't have enough time to take care of promtool
currently.
Julien Pivotto (<roidelapluie@prometheus.io> / @roidelapluie) is the main/default maintainer, some parts of the codebase have other maintainers:
-* `cmd`
- * `promtool`: Simon Pasquier (<pasquier.simon@gmail.com> / @simonpasquier)
* `discovery`
* `k8s`: Frederic Branczyk (<fbranczyk@gmail.com> / @brancz)
* `documentation`
