Entity names should be hidden on line charts with multiple indicators when only one entity is chosen

[pabloarosado]

Description

Entity names should be hidden on line charts with multiple indicators when only one entity is chosen. Currently, this is what happens when "User can change entity" is selected (since there can only be one entity). However, when "User can add and remove data" is selected, the entity name appears, even when only one entity is selected.

I suppose there is no point in showing the entity name if only one entity is selected, but maybe there are good cases for that behavior that I'm not aware of right now.

Steps to reproduce

Steps to reproduce the behavior:

1. Go to any chart with multiple indicators, e.g. this chart: https://admin.owid.io/admin/charts/8482/edit
2. Go to the Data tab and change from "User can change entity" to "User can add and remove data".
3. See that now "United Kingdom" appears next to the lines, even though there is no other entity selected.

Additional context

Fixing this would be very helpful for an article on air pollution that relies on the air pollution data explorer: on certain views, "World" appear multiple times, which is quite ugly. See discussion on Slack.