When building AssetBundles, if you remove a bundle from the project entirely, it will still appear in the manifest after an incremental build if there are no other AssetBundle changes to build. Removing a bundle should always remove it from the manifest in the next asset bundle build, even if there are no other changes.

Reproducible on 5.0, 5.1 versions.

Repro steps:
1. open project "demo.zip".

2. Start by building AssetBundles(menu bar AssetBundles -> Build AssetBundles). Before, check if cube's asset has been set up with an asset bundle name;

3. Observe that AssetBundle cube.unity3d has been created.

4. On the cube asset, assign it to AssetBundle 'None' and then choose Remove Unused names. This should completely remove the cube.unity3d AssetBundle from the project.

5. Build asset bundles again. This will result in the output "No asset bundle needs to be rebuilt for this build."

6. Observe that the built cube.unity3d AssetBundle is still present, and still appears in the AssetBundleManifest.

7. Make a change that will result in an AssetBundle change (for example, change a color on MyMaterial).
Build bundles again. Observe that cube.unity3d is no longer listed in the manifest. However, the file still exists in the output directory.

This is OK, as I can then use the manifest to reconcile the output directory and delete old bundles that no longer belong.