Remove irrelevant api.Window.beforeunload_event.custom_text_support feature (#11753)
Remove irrelevant api.Window.beforeunload_event.custom_text_support feature (#11753)

This PR removes the irrelevant `beforeunload_event.custom_text_support` member of the `Window` API as per the corresponding [data guidelines](https://github.com/mdn/browser-compat-data/blob/main/docs/data-guidelines.md#removal-of-irrelevant-features). The lack of current support has been confirmed by the [mdn-bcd-collector](https://mdn-bcd-collector.appspot.com) project (v3.2.2), even if the current BCD suggests support.
chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data