remove `xcshareddata` from gitignore (#27563)

Summary:
Files that are inside `xcshareddata` are meant to be "shared", and that means committed to git. These files are for example `ios/RnDiffApp.xcodeproj/xcshareddata/xcschemes/RnDiffApp.xcscheme`, and if ignored that means they are removed from the repo or are missing when cloning. These should be present. They are the shared schemes so Xcode knows what and how to build, on everyone's machine.

I noticed it being there in a new project on 0.62.0-rc.0.

## Changelog

[iOS] [Changed] - Remove the xcshareddata from .gitignore
Pull Request resolved: https://github.com/facebook/react-native/pull/27563

Differential Revision: D19166364

Pulled By: TheSavior

fbshipit-source-id: 0377dc5b08ba04d121f9ed5e4027942a7a9f463f

template/_gitignore

@@ -20,7 +20,6 @@ DerivedData
 *.hmap
 *.ipa
 *.xcuserstate
-xcshareddata
 
 # Android/IntelliJ
 #