refactor: remove unnecessary arrow function wrap

- add installed handler

content/install-hook-openusercss.js

@@ -30,8 +30,7 @@
     });
   };
 
-  const attachInstalledListeners = () => {
-    window.addEventListener('message', event => {
+  const installedHandler = event => {
     if (
       event.data
       && event.data.type === 'ouc-is-installed'
@@ -53,7 +52,10 @@
         sendInstalledCallback(callbackObject);
       });
     }
-    });
+  };
+
+  const attachInstalledListeners = () => {
+    window.addEventListener('message', installedHandler);
   };
 
   const doHandshake = () => {
@@ -102,9 +104,7 @@
     });
   };
 
-  const attachHandshakeListeners = () => {
-    // Wait for the handshake request, then start it
-    window.addEventListener('message', event => {
+  const handshakeHandler = event => {
     if (
       event.data
       && event.data.type === 'ouc-handshake-question'
@@ -112,7 +112,11 @@
     ) {
       doHandshake();
     }
-    });
+  };
+
+  const attachHandshakeListeners = () => {
+    // Wait for the handshake request, then start it
+    window.addEventListener('message', handshakeHandler);
   };
 
   const sendInstallCallback = data => {
@@ -124,9 +128,7 @@
     });
   };
 
-  const attachInstallListeners = () => {
-    // Wait for an install event, then save the theme
-    window.addEventListener('message', event => {
+  const installHandler = event => {
     if (
       event.data
       && event.data.type === 'ouc-install-usercss'
@@ -144,13 +146,19 @@
         });
       });
     }
-    });
   };
 
-  (() => {
+  const attachInstallListeners = () => {
+    // Wait for an install event, then save the theme
+    window.addEventListener('message', installHandler);
+  };
+
+  window.removeEventListener('message', installHandler);
+  window.removeEventListener('message', handshakeHandler);
+  window.removeEventListener('message', installedHandler);
+
   attachHandshakeListeners();
   attachInstallListeners();
   attachInstalledListeners();
   askHandshake();
 })();
-})();