Revert "Add knowledge about eap-1.3 channel to plugin (KT-25414)"

It's decided to migrate to scheme with two predefined channels:
`eap` and `eap-next`.

This reverts commit 4f1f1e6

idea/src/org/jetbrains/kotlin/idea/configuration/KotlinLanguageConfiguration.kt

@@ -25,16 +25,15 @@ class KotlinUpdatesSettingsConfigurable : SearchableConfigurable, Configurable.N
                 it.startsWith("https://plugins.jetbrains.com/plugins/") &&
                         (it.endsWith("/6954") || it.endsWith(KotlinPluginUtil.KOTLIN_PLUGIN_ID.idString))
             }
-
-            EAPChannels.values().find { it.uiIndex == channel }?.let { eapChannel ->
-                hosts.add(eapChannel.url)
+            when (channel) {
+                EAPChannels.EAP_1_3.uiIndex -> hosts.add(EAPChannels.EAP_1_3.url)
+                EAPChannels.EAP_1_2.uiIndex -> hosts.add(EAPChannels.EAP_1_2.url)
             }
         }
 
         enum class EAPChannels(val url: String, val uiIndex: Int) {
             EAP_1_2("https://plugins.jetbrains.com/plugins/eap-1.2/${KotlinPluginUtil.KOTLIN_PLUGIN_ID.idString}", 1),
-            EAP_1_3("https://plugins.jetbrains.com/plugins/eap-1.3/${KotlinPluginUtil.KOTLIN_PLUGIN_ID.idString}", 2),
-            EAP_NEXT("https://plugins.jetbrains.com/plugins/eap-next/${KotlinPluginUtil.KOTLIN_PLUGIN_ID.idString}", 3);
+            EAP_1_3("https://plugins.jetbrains.com/plugins/eap-next/${KotlinPluginUtil.KOTLIN_PLUGIN_ID.idString}", 2);
 
             private val hasChannel: Boolean get() = url in UpdateSettings.getInstance().pluginHosts
 
@@ -111,12 +110,7 @@ class KotlinUpdatesSettingsConfigurable : SearchableConfigurable, Configurable.N
             }
         }
 
-        savedChannel = EAPChannels.values().asSequence()
-            .map { eapChannel -> eapChannel.indexIfAvailable() }
-            .filterNotNull()
-            .firstOrNull()
-                ?: 0
-
+        savedChannel = EAPChannels.EAP_1_3.indexIfAvailable() ?: EAPChannels.EAP_1_2.indexIfAvailable() ?: 0
         form.channelCombo.selectedIndex = savedChannel
 
         form.channelCombo.addActionListener {

idea/src/org/jetbrains/kotlin/idea/configuration/ui/ConfigurePluginUpdatesForm.form

@@ -85,7 +85,6 @@
                 <item value="Stable"/>
                 <item value="Early Access Preview 1.2.x"/>
                 <item value="Early Access Preview 1.3"/>
-                <item value="Early Access Preview (Next)"/>
               </model>
             </properties>
           </component>