Remove Swift plugin on 5.X banches

[johnsudaar]

Swift plugin is not (and will not) be available on ES 5.X versions (see: BigDataBoutique/elasticsearch-repository-swift#3 (comment))

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[elasticmachine]

Pinging @elastic/es-docs

[cbuescher]

Hi @johnsudaar,

thanks for the update, looking at the discussion it makes sense to me to remove the plugin from the 5.x docs. Do you think it also makes sense to update the Link to point to https://github.com/BigDataBoutique/elasticsearch-repository-swift for later versions, starting with 6.0? Looking at the repo I'm not sure if the plugin supports 6.x versions yet.
I will go ahead and merge this change to the 5.x branches, let me know if you think this warrants a follow-up agains the current master and 6.x branches too.