[swift4] The podspec docset_url attribute has been removed from the CocoaPods...

[crumpf]

…Specification DSL, so the podDocsetURL swift4 config option can be removed.

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• ✅ Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Issue: #8315 : Remove podDocsetURL config option for swift4

There is a config option in the swift4 language, podDocsetURL, which is used to specify a docset_url attribute in the generated podspec. docset_url has been removed from the CocoaPods Specification DSL (CocoaPods/Core#284), so this config option can be removed from swagger-codegen.

/cc swift technical committee: @ehyche