Remove error handling in clientCutText()

It is not necessary as Websock.flush() is guaranteed to succeed and
give us some space. It also remove the call to _fail(), which was
invalid at this place as clientCutText() is not a method on RFB.

kasmweb/core/rfb.js

@@ -2024,11 +2024,6 @@ RFB.messages = {
         while (remaining > 0) {
 
             let flushSize = Math.min(remaining, (sock._sQbufferSize - sock._sQlen));
-            if (flushSize <= 0) {
-                this._fail("Clipboard contents could not be sent");
-                break;
-            }
-
             for (let i = 0; i < flushSize; i++) {
                 buff[sock._sQlen + i] =  text.charCodeAt(textOffset + i);
             }