Fix popup not opening (#8314)

MetaMask would sometimes get into a state where the notification popup
would never open. This could happen if the notification window was
closed shortly after being opened. After this happened, no popups would
show up until after the extension was reset.

This was happening because the background thought the popup was already
open. The variable it uses to track whether the popup was open or not
was being set to `true` immediately after the background asked the
browser to open a new window, before a handler was attached that could
respond to the window being closed.

Removing this line seems to solve the problem.

This line was added originally in #5437, which dealt with batch
transactions. Batches of transactions seem to work just fine without
this line though (from local testing), and I can't think of why this
would be required.

Closes #7051

app/scripts/background.js

@@ -460,7 +460,6 @@ function triggerUi () {
     const currentlyActiveMetamaskTab = Boolean(tabs.find((tab) => openMetamaskTabsIDs[tab.id]))
     if (!popupIsOpen && !currentlyActiveMetamaskTab && !notificationIsOpen) {
       notificationManager.showPopup()
-      notificationIsOpen = true
     }
   })
 }