deps: Remove direct eslint-config-airbnb-base dependency.

We don't ever import this ourselves, and `eslint-config-airbnb`
already has it, since it's listed as a `dependency` by that package.

It's never been listed there as a peer dependency, even as far back
as v15.0.2 [1], the version we were on when we added
`eslint-config-airbnb-base` as a direct dependency, in 984f771. A
listing in `peerDependencies` would be a compelling reason for us to
depend on it directly [2].

Moreover, 984f771 didn't explain the addition, and the main focus
of that commit was elsewhere, so it seems unlikely that it was
well-considered.

So, remove it. This will simplify ESLint upgrades, like the one
later in this series.

Also, run `yarn yarn-deduplicate && yarn`, as prompted by
`tools/test deps`.

[1]: https://github.com/airbnb/javascript/blob/c68fa632624f088b4bbe81192a50554b0c6af970/packages/eslint-config-airbnb/package.json

[2]: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/unimodules.20unmet.20peer.20dependency/near/850108

package.json

@@ -97,7 +97,6 @@
     "deep-freeze": "^0.0.1",
     "eslint": "^5.16.0",
     "eslint-config-airbnb": "^17.1.1",
-    "eslint-config-airbnb-base": "^13.2.0",
     "eslint-config-prettier": "^6.0.0",
     "eslint-plugin-flowtype": "^3.9.0",
     "eslint-plugin-import": "^2.18.1",