remove BuildTvos method from Firestore/Editor/Builder.cs (#541)

Remove an attempt to support tvOS for Firestore in the Editor/Builder.cs code. This failed to compile due to Unity not being able to resolve the tvOSSdkVersion enumerated type, for some reason.

However, it seems that these methods aren't invoked anywhere. Removing for now. Will add back and fix if Firestore fails to build for tvOS in future testing.

firestore/testapp/Assets/Firebase/Editor/Builder.cs

@@ -43,23 +43,6 @@ public static void BuildIos() {
     Build(options);
   }
 
-  public static void BuildTvos() {
-    var options = new BuildPlayerOptions();
-
-    // For tvOS, this is the name of the folder containing the generated XCode
-    // project.
-    options.locationPathName = "tvos-build";
-    options.target = BuildTarget.tvOS;
-    // Firebase Unity plugins don't seem to work on a simulator.
-    PlayerSettings.tvOS.sdkVersion = tvOSSdkVersion.DeviceSDK;
-
-    // AcceptExternalModificationsToPlayer corresponds to "Append" in the Unity
-    // UI -- it allows doing incremental builds.
-    options.options = BuildOptions.AcceptExternalModificationsToPlayer;
-
-    Build(options);
-  }
-
   public static void BuildAndroid() {
     var options = new BuildPlayerOptions();