misc: Remove URI check from EmbedBuilder (#1778)

`Uri.IsWellFormedUriString()` doesn't return the expected result for specific urls, removed until the DotNet team actually resolves it ( https://github.com/dotnet/runtime/issues/21626 )

src/Discord.Net.Core/Extensions/StringExtensions.cs

@@ -1,10 +0,0 @@
-using System;
-
-namespace Discord
-{
-    internal static class StringExtensions
-    {
-        public static bool IsNullOrUri(this string url) =>
-            string.IsNullOrEmpty(url) || Uri.IsWellFormedUriString(url, UriKind.Absolute);
-    }
-}