refactor(ios): remove allowFileAccessFromFileURLs preference call

[jcesarmobile]

allowFileAccessFromFileURLs is a non documented preference that can cause security warnings
it allows to load file:// urls in the WKWebView, but Capacitor uses custom schemes for serving the local files and provides a helper to convert any file:// url to a capacitor://localhost url, so the preference shouldn't be needed anymore, and if anybody is using it, should move to the capacitor://localhost urls

closes #6131

[markemer]

Yeah, this isn't guaranteed to stick around and if Apple's having a bad day they might even reject and app with it, although, I doubt it. Seems like we're using the proper alternative anyway.