Remove experimental rancherd asset

[deniseschannon]

Currently, Rancher ships with a rancherd asset which was an experimental way of installing Rancher introduced in Rancher v2.5.

As we are looking to define the exact use case and implementation of this asset, we have decided it needs to be updated and re-architected and are looking to remove the experimental ability of this installation.

[sowmyav27]

To validate:

On an RC, verify the RancherD assets is not generated.

[cbron]

Only deleting asset from the release objects, nothing else in the code.

[jiaqiluo]

The build process should not publish any rancherd assets now in the master branch

[brendarearden]

Validation: PASSED

Steps to Reproduce:

1. Go to a previous release tag, in this case v2.5.9.
2. Check the assets list for rancherd-amd64 and rancherd-amd64.tar.gz
3. See that the assets are available in the Assets list.

Steps to Test:

1. Go to the v2.6.0-rc1 tag.
2. Check the assets list for rancherd-amd64 and rancherd-amd64.tar.gz
3. See that the assets are no longer list in the Assets list.