remove dockerd man-page (moved back to moby repository)

When splitting the docker CLI from the moby/moby repository, the dockerd
documentation and man-pages were moved to the CLI repository. This was a
bit of a bad choice, as now the documentation and code lived in separate
repositories, but when the CLI and dockerd packages were split, the man
page for dockerd ended up in the CLI package.

Starting with [moby@d6e9b5f], the dockerd man-page has been reintegrated
into the moby repository, so we cab  remove it from the CLI repository.

[moby@d6e9b5f]: moby/moby@d6e9b5f

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah