xds: Return nil error after context cancel

Do not return an error from xds server when the context is cancelled, as
this is part of normal operation, and we test for this in
server_e2e_test.

This resolves a test flake:

panic: Fail in goroutine after  has completed

Signed-off-by: Jarno Rajahalme <jarno@isovalent.com>

pkg/envoy/xds/server.go

@@ -262,7 +262,7 @@ func (s *Server) processRequestStream(ctx context.Context, streamLog *logrus.Ent
 		switch chosen {
 		case doneChIndex: // Context got canceled, most likely by the client terminating.
 			streamLog.WithError(ctx.Err()).Debug("xDS stream context canceled")
-			return ctx.Err()
+			return nil
 
 		case reqChIndex: // Request received from the stream.
 			if !recvOK {