Remove injected script tags after use

app/scripts/contentscript.js

@@ -37,8 +37,10 @@ function injectScript (content) {
   try {
     const container = document.head || document.documentElement
     const scriptTag = document.createElement('script')
+    scriptTag.setAttribute('async', false)
     scriptTag.textContent = content
     container.insertBefore(scriptTag, container.children[0])
+    container.removeChild(scriptTag)
   } catch (e) {
     console.error('MetaMask script injection failed', e)
   }