Summary: I found that clang-format config file allows to specify rules on a per-language basis, so I moved Objective-C specific rules to the unified config. Now we have only one clang-format file. Yay!

Reviewed By: JoshuaGross

Differential Revision: D14018902

fbshipit-source-id: 45c1e185b8f2b8151ea202b3d9a68a3886597198