[objective-c/en] Remove non-ARC examples

ARC is and has been the standard for a while now (since Xcode 4.2). Including non-ARC examples is - IMHO - confusing and of very little value to newcomers who will most likely never work on non-ARC code. I propose to remove all non-ARC related content from the Objective-C file and I'd be happy to do the work and put up a PR. Thoughts? @YannickL @levibostian @cwalk

Start with the repository's Objective-C file referenced in the issue and identify its non-ARC examples and explanations. Remove the non-ARC-related content, then review the document to confirm that the remaining material is focused on ARC and still reads coherently.

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