Rejected Transaction should be removed from unapprovedTxs

[tmashuang]

There is some cases where rejected/cancelled transactions do not get out of the unapprovedTxs list, mostly happens when rejecting a transaction supplied by a Dapp. This is not a consistent issue,

"unapprovedTxs": {
 "3239248756227922": {
   "id": 3239248756227922,
   "time": 1522948634588,
   "status": "rejected",
   "metamaskNetworkId": "4",
   "loadingDefaults": false,
   "txParams": {
     "from": "address",
     "to": "address",
     "data": "data hash",
     "gasPrice": "0x4a817c800",
     "description": "blah blah blah",
     "name": "name",
     "gas": "0x75c07",
     "value": "0x0"
   },
  }
}

[bdresser]

fixed in #4667