chore: Remove obsolete keys

Remove references to obsolete keys.
* PUBNUB_*: These were used for the old mobile state sync feature,
which was removed a long time ago.
* Etherscan: This was used for incoming transactions, but we've since
switched to our own API.
* Opensea: We've switched to Reservoir

app/scripts/metamask-controller.js

@@ -761,8 +761,6 @@ export default class MetamaskController extends EventEmitter {
         }),
     });
 
-    this.nftController.setApiKey(process.env.OPENSEA_KEY);
-
     const nftDetectionControllerMessenger =
       this.controllerMessenger.getRestricted({
         name: 'NftDetectionController',

builds.yml

@@ -214,13 +214,9 @@ env:
   # API keys to 3rd party services
   ###
 
-  - PUBNUB_PUB_KEY: null
-  - PUBNUB_SUB_KEY: null
   - SEGMENT_HOST: null
   - SENTRY_DSN: null
   - SENTRY_DSN_DEV: null
-  - OPENSEA_KEY: null
-  - ETHERSCAN_KEY: null
   # also INFURA_PROJECT_ID below
 
   ###
@@ -318,3 +314,10 @@ env:
   # This should NEVER be enabled in production since it slows down react
   ###
   - ENABLE_WHY_DID_YOU_RENDER: false
+
+  ###
+  # Unused environment variables referenced in dependencies
+  # Unset environment variables cause a build error. These are set to `null` to tell our build
+  # system that they are intentionally unset.
+  ###
+  - ETHERSCAN_KEY: null # Used by `gridplus-sdk/dist/util.js`