Fix SignalR union workaround resolver failing on multiple union'd types

[smoogipoo]

Without this, when adding a new union'd type, Messagepack throws the following at startup:

[network] 2022-02-24 11:26:32 [verbose]: SpectatorClient connection error: System.TypeInitializationException: The type initializer for 'osu.Game.Online.SignalRUnionWorkaroundResolver' threw an exception.
[network] 2022-02-24 11:26:32 [verbose]: ---> System.ArgumentException: An item with the same key has already been added. Key: osu.Game.Online.Multiplayer.MatchUserRequest
[network] 2022-02-24 11:26:32 [verbose]: at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
[network] 2022-02-24 11:26:32 [verbose]: at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
[network] 2022-02-24 11:26:32 [verbose]: at System.Collections.Generic.Dictionary`2.AddRange(IEnumerable`1 collection)
[network] 2022-02-24 11:26:32 [verbose]: at System.Collections.Generic.Dictionary`2..ctor(IEnumerable`1 collection, IEqualityComparer`1 comparer)
[network] 2022-02-24 11:26:32 [verbose]: at System.Collections.Generic.Dictionary`2..ctor(IEnumerable`1 collection)
[network] 2022-02-24 11:26:32 [verbose]: at osu.Game.Online.SignalRUnionWorkaroundResolver.createFormatterMap() in /home/smgi/Repos/osu/osu.Game/Online/SignalRUnionWorkaroundResolver.cs:line 32
[network] 2022-02-24 11:26:32 [verbose]: at osu.Game.Online.SignalRUnionWorkaroundResolver..cctor() in /home/smgi/Repos/osu/osu.Game/Online/SignalRUnionWorkaroundResolver.cs:line 22

[frenzibyte]

Right, the Distinct used to be there but I have removed it without further thought on a "many types to one base type" scenario (see #15684 (comment)).