Remove dummy plugin

There's a more up-to-date version in the rrdd-plugin library.

Signed-off-by: John Else <john.else@citrix.com>
Remove unneeded module open

Signed-off-by: John Else <john.else@citrix.com>
Remove Rrdp_common and switch to Rrdd_plugin

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #15 from johnelse/use-ocaml-rrdd-plugin

Remove Rrdp_common and Rrdp_dummy, switch to Rrdd_plugin