Remove unused isPopup state (#8280)

This property was being set upon each change to background state.

app/scripts/background.js

@@ -100,7 +100,6 @@ initialize().catch(log.error)
  * @property {boolean} isInitialized - Whether the first vault has been created.
  * @property {boolean} isUnlocked - Whether the vault is currently decrypted and accounts are available for selection.
  * @property {boolean} isAccountMenuOpen - Represents whether the main account selection UI is currently displayed.
- * @property {boolean} isPopup - Returns true if the current view is an externally-triggered notification.
  * @property {string} rpcTarget - DEPRECATED - The URL of the current RPC provider.
  * @property {Object} identities - An object matching lower-case hex addresses to Identity objects with "address" and "name" (nickname) keys.
  * @property {Object} unapprovedTxs - An object mapping transaction hashes to unapproved transactions.

development/states/navigate-txs.json

@@ -3,7 +3,6 @@
     "isInitialized": true,
     "isUnlocked": true,
     "isAccountMenuOpen": false,
-    "isPopup": false,
     "rpcTarget": "https://rawtestrpc.metamask.io/",
     "identities": {
       "0x8cf82b5aa41ff2282427be151dd328568684007a": {

ui/app/ducks/metamask/metamask.js

@@ -1,13 +1,10 @@
 import { actionConstants as actions } from '../../store/actions'
-import { getEnvironmentType } from '../../../../app/scripts/lib/util'
-import { ENVIRONMENT_TYPE_POPUP } from '../../../../app/scripts/lib/enums'
 
 export default function reduceMetamask (state = {}, action) {
   const metamaskState = Object.assign({
     isInitialized: false,
     isUnlocked: false,
     isAccountMenuOpen: false,
-    isPopup: getEnvironmentType() === ENVIRONMENT_TYPE_POPUP,
     rpcTarget: 'https://rawtestrpc.metamask.io/',
     identities: {},
     unapprovedTxs: {},