Remove old plugins directory

The two hooks still in here will be ported to their respective
repository, as they are not workflow-specific.

Change-Id: Ic1a16d808bfb8326dca601f8859b005453c0e5eb
Reviewed-on: http://gerrit.causes.com/35607
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>