Remove some unused mailer dependencies

packages/inbound-email-handler/package.json

@@ -29,10 +29,8 @@
     "@google-cloud/functions-framework": "1.9.0",
     "@google-cloud/pubsub": "^2.18.4",
     "@sendgrid/client": "^7.6.0",
-    "@sendgrid/inbound-mail-parser": "^7.6.0",
     "@sentry/serverless": "^6.16.1",
     "axios": "^0.21.1",
-    "json-bigint": "^1.0.0",
     "parse-headers": "^2.0.4",
     "parse-multipart-data": "^1.2.1",
     "quoted-printable": "^1.0.1"