Add missing retrieveMessage methods to ReactionEvents

[Kaktushose]

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Description

Adds a retrieveMessage method to the MessageReactionRemoveAllEvent and MessageReactionRemoveEmojiEvent. While it's just a shortcut for getChannel().retrieveMessageById(getMessageId()) and can therefore be seen as redundant and syntatic sugar, the GenericMessageReactionEvent also has such a method. So this addition makes the API more consistent

[duncte123]

Wouldn't it be better to move this to the GenericMessageEvent class instead? Seems right now that you are just repeating the same code a few times

[Kaktushose]

No, because some subclasses like MessageDeleteEvent couldn't use this method

[yoyosource]

What about adding an interface like GenericMessageReactionRemoveEvent with the common code then?