Remove documentation about tvOS targets

[Craig Coles]

Describe the solution you'd like

The default react-native init now comes with tvOS targets... These add un-needed cruft to the project. Best plan is to open the XCode project, remove the tvOS targets and then delete the files in the project themselves.

This is no longer the case as of v0.62, which can be found in the v0.62 release notes.

If tvOS is needed, then you can install the react-native-tvos package.