Remove the (un)wrap(In|Out)bound(In|Out) customisation points

[weissi]

_EmittingChannelHandler, ChannelInboundHandler and ChannelOutboundHandler define bogus customisation points

• func wrapOutboundOut(_ value: OutboundOut) -> NIOAny
• func unwrapInboundIn(_ value: NIOAny) -> InboundIn
• func wrapInboundOut(_ value: InboundOut) -> NIOAny
• func unwrapOutboundIn(_ value: NIOAny) -> OutboundIn

they only have one correct implementation so they shouldn't be customisation points. This will also come with a (small) perf overhead.

Anyway, once possible we should remove them.

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Given that they only have one correct implementation, we could actually just remove them without a SemVer major too. But as @Lukasa points out, some people seem to be using NIO in -enable-library-evolution mode where this would be an issue. So let's just leave it, the overhead is small and folks should use Self.(un)wrap(In|Out)bound(In|Out) anyway which doesn't have this issue.