Get rid of $legacyMapping
Get rid of $legacyMapping

We now have 'special' in $formatters, there's no need to keep
the mapping to the legacy formatter around.

Change-Id: I66f330e8c84a50858658361caef521a3e5717d58

wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo,wikimedia/mediawiki-extensions-Echo