[SPARK-18762][WEBUI] Web UI should be http:4040 instead of https:4040

## What changes were proposed in this pull request?

When SSL is enabled, the Spark shell shows:
```
Spark context Web UI available at https://192.168.99.1:4040


```
This is wrong because 4040 is http, not https. It redirects to the https port.
More importantly, this introduces several broken links in the UI. For example, in the master UI, the worker link is https:8081 instead of http:8081 or https:8481.

CC: mengxr liancheng

I manually tested accessing by accessing MasterPage, WorkerPage and HistoryServer with SSL enabled.

Author: sarutak <sarutak@oss.nttdata.co.jp>

Closes #16190 from sarutak/SPARK-18761.

(cherry picked from commit bb94f61a)
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -187,8 +187,7 @@ private[deploy] class Worker(
     webUi = new WorkerWebUI(this, workDir, webUiPort)
     webUi.bind()
 
-    val scheme = if (webUi.sslOptions.enabled) "https" else "http"
-    workerWebUiUrl = s"$scheme://$publicAddress:${webUi.boundPort}"
+    workerWebUiUrl = s"http://$publicAddress:${webUi.boundPort}"
     registerWithMaster()
 
     metricsSystem.registerSource(workerSource)

core/src/main/scala/org/apache/spark/ui/WebUI.scala

@@ -147,10 +147,7 @@ private[spark] abstract class WebUI(
   }
 
   /** Return the url of web interface. Only valid after bind(). */
-  def webUrl: String = {
-    val protocol = if (sslOptions.enabled) "https" else "http"
-    s"$protocol://$publicHostName:$boundPort"
-  }
+  def webUrl: String = s"http://$publicHostName:$boundPort"
 
   /** Return the actual port to which this server is bound. Only valid after bind(). */
   def boundPort: Int = serverInfo.map(_.boundPort).getOrElse(-1)