Allow do disable the "Override" function in context menu for asset

It's good catch to remove override if it's useless.
But i think better to pass some real asset options with _assets_get_overridable_fields() to JS instead of adding some option.
In this approach — if there is no fields to be overridden, override contextual option will be hidden.
So my idea is:
- during provisioning of field config on Drupal side check if there are field to override with _assets_get_overridable_fields()
- get this option in initialization of CKEditor menu and commands and do not init override

Hi, great work!

But i'm still thinking that we need to provide a generic way to configure contextual options in such case, instead of configuring each item separately.

About hiding override option automatically — it's good idea.
Just small detail. From what i see, editor.contextMenu.addListener callback called each time when we open menu.
Result of callback is the list of menu items for each certain situation (asset). This means that instead of deletion of previously added item (couple of lines above), we need to just do not add it.
So what we need to do is just to add new condition here:

if (conf && conf.modes && !(conf.modes.length === 1 && conf.modes.full && !conf.fields.length)) {

I propose to split your patch into 2 — fix with auto-hiding and configurable option — to make possible for others to re-use this feature if needed.

From my point of view, the most acceptable solution is next.

We could add to place where we build context menu additional check, something like (it's rough example and should be well tested):

if (!element.data('asset-suppress-override')) {//...}
...
if (!element.data('asset-suppress-edit')) {//...}

It will allow:
1. Do not break any previous versions/setups. No any hook_updates needed.
2. Allow any custom developers in a flexible fashion to control the context menu even per certain asset instance if needed.
I.e. we could override theming of asset template per asset type and restrict it. Or to add this attribute to global asset template to restrict options at all. Or these attributes could be added on the fly via JS if needed.

In the future, we could improve this and make it dynamic, i.e.
<some-asset-tag-in-template data-asset-suppress-menus="assetcut,assetdelete,assetoverride, etc">

So in your case, you just need to modify asset template to add attribute where needed.

Great work!
Committed to dev.