Remove duplicate showWindow method (#1918)

main.js

@@ -41,6 +41,10 @@ const config = require("./app/config");
 const userConfig = require('./app/user_config');
 
 function showWindow() {
+  if (!mainWindow) {
+    return;
+  }
+
   // Using focus() instead of show() seems to be important on Windows when our window
   //   has been docked using Aero Snap/Snap Assist. A full .show() call here will cause
   //   the window to reposition:
@@ -310,12 +314,6 @@ function showDebugLog() {
   }
 }
 
-function showWindow() {
-  if (mainWindow) {
-    mainWindow.show();
-  }
-};
-
 function openReleaseNotes() {
   shell.openExternal('https://github.com/WhisperSystems/Signal-Desktop/releases/tag/v' + app.getVersion());
 }