Drop Debug.fail

...from `release-2.5`.  I'm not 100% confident it won't be hit and we'll
recover if we do.  It's still in master, so dogfooding should flush out
any remaining issues before 2.6.

Author: amcasey

src/server/server.ts

@@ -432,9 +432,6 @@ namespace ts.server {
                 if (this.activeRequestCount > 0) {
                     this.activeRequestCount--;
                 }
-                else {
-                    Debug.fail("Received too many responses");
-                }
 
                 while (this.requestQueue.length > 0) {
                     const queuedRequest = this.requestQueue.shift();