Currently, any default drop effect is "move" on editable content even when
the editor cannot remove the source element (e.g., outside of any editing host).
The drop effect is initialized by `nsContentUtils::SetDataTransferInEvent()`
which is called when `EditorEventListener::CanDrop()` and it returns true.
However, it requires unnecessary cost if we make `nsContentUtils` check
whether the dragging element is removable. Therefore, we should make
`EditorEventListener` overwrites it with checking whether the source node is
editable or not.
Differential Revision: https://phabricator.services.mozilla.com/D57446
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extra : moz-landing-system : lando
f38bb2c653