Streamline and update `Rakefile`

[mokagio]

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While working on #18845 and #18850, I noticed a few things that could be improved in the Rakefile:

• DRY how we load Podfile.lock
• We have tasks such as test and clean that are redundant given our use of Fastlane. We should either remove them or make them run the appropriate Faslane lane under the hood.

https://github.com/wordpress-mobile/WordPress-iOS/blob/2185a1e1c2cdaa0b089e93ba75b315e058f360ce/Rakefile#L216-L224

• Same goes for this custom xcodebuild call. Fastlane can do this for us with extra parameter and safety checks.

https://github.com/wordpress-mobile/WordPress-iOS/blob/2185a1e1c2cdaa0b089e93ba75b315e058f360ce/Rakefile#L732-L743

Also note that xcodebuild call will likely fail, because "iPhone 6s" is no longer one of the Simulator that ship with Xcode out of the box:

[peril-wordpress-mobile[bot]]

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