Remove workaroudn for linker issue in Http3RequestStream (#57571)

Part of the linker issue dotnet/linker#2181 has been fixed in dotnet/linker#2205. This part was the one affecting Http3RequestStream.

This change simple reverts the workaround since it's not needed anymore.

Author: vitek-karas

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs

@@ -127,13 +127,6 @@ public async Task<HttpResponseMessage> SendAsync(CancellationToken cancellationT
 
             try
             {
-                // Works around linker issue where it tries to eliminate QuicStreamAbortedException
-                // https://github.com/dotnet/runtime/issues/57010
-                #if TARGET_MOBILE
-                if (string.Empty.Length > 0)
-                    throw new QuicStreamAbortedException("", 0);
-                #endif
-
                 BufferHeaders(_request);
 
                 // If using Expect 100 Continue, setup a TCS to wait to send content until we get a response.