Clean duplicates off sites.json

From @eerovaher (https://github.com/astropy/astropy/pull/12721/files#r786773846)

Raising warnings about duplicated names sounds like a good idea, but it should not be implemented until the `sites.json` file in `astropy-data` is cleaned from duplicates, otherwise the users would receive many warnings that they can't really do anything about. Removing the duplicates from `sites.json` would reduce the number of usable keys for anyone not using the bleeding edge version of `astropy`, so it doesn't seem it would be a good idea to do that quite yet.

When it comes to checking for duplicated names for different sites then it wouldn't be too difficult to add a test to `astropy-data` that would check for that. The output of `astropy.coordinates.EarthLocation.get_site_names()` shows that currently there aren't any duplicated labels across sites other than the empty string that this pull request takes care of.

Inspect sites.json in the astropy-data repository and identify the duplicate site names referenced by astropy.coordinates.EarthLocation.get_site_names(). Check the related astropy-data context before changing entries, since the issue notes that removing duplicates could reduce usable keys; done means the data has no unintended duplicate names without breaking those keys.

Written by the indexing model from the issue text.