Start to remove old things in Maui

[mattleibow]

Description of Change

Removing some old things in maui that are no longer needed/used. This is part of #1965

Removed from IPlatformServices:

• string GetHash(string input)
• Task<Stream> GetStreamAsync(Uri uri, CancellationToken cancellationToken)
• IIsolatedStorageFile GetUserStoreForApplication() (and all isolated storage usages)
• void OpenUriAction(Uri uri)

Changed in Forms:

• Obsoleted void OpenUriAction(Uri uri) in favor of Essentials
• Removed void SetFlags(params string[] flags)
• Removed IReadOnlyList<string> Flags { get; }

Removed from Device:

• void SetFlags(params string[] flags)
• IReadOnlyList<string> Flags { get; }