Removes duplicate imports of okhttp3.ResponseBody.
Instead of adding the import for every operation, this sets a "global" variable isResponseFile as already expected by api.mustache.

Adding the import for every operation (that has isResponseFile set to true) was introduced in #9239 and lead to non-compiling generated Apis as multiple imports are not allowed.

This fixes #9682.

@jimschubert can you review this?

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