Remove irrelevant api.Text.isElementContentWhitespace feature (#11859)
Remove irrelevant api.Text.isElementContentWhitespace feature (#11859)

This PR removes the irrelevant `isElementContentWhitespace` member of the `Text` 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