Remove some unused Babel plugins (#157)

We brought these in when we removed preset-stage-1, but most of them
aren't needed. The only one I think we actually need is class properties
because we have some tests that rely on it.

.babelrc

@@ -14,11 +14,5 @@
   ],
   "plugins": [
     "@babel/plugin-proposal-class-properties",
-    "@babel/plugin-proposal-do-expressions",
-    "@babel/plugin-proposal-export-default-from",
-    "@babel/plugin-proposal-logical-assignment-operators",
-    ["@babel/plugin-proposal-nullish-coalescing-operator", { "loose": false }],
-    ["@babel/plugin-proposal-optional-chaining", { "loose": false }],
-    ["@babel/plugin-proposal-pipeline-operator", { "proposal": "minimal" }]
   ]
 }

package.json

@@ -36,13 +36,6 @@
     "@babel/generator": "^7.0.0",
     "@babel/plugin-external-helpers": "^7.0.0",
     "@babel/plugin-proposal-class-properties": "^7.0.0",
-    "@babel/plugin-proposal-do-expressions": "^7.0.0",
-    "@babel/plugin-proposal-export-default-from": "^7.0.0",
-    "@babel/plugin-proposal-logical-assignment-operators": "^7.0.0",
-    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.0.0",
-    "@babel/plugin-proposal-object-rest-spread": "^7.0.0",
-    "@babel/plugin-proposal-optional-chaining": "^7.0.0",
-    "@babel/plugin-proposal-pipeline-operator": "^7.0.0",
     "@babel/plugin-transform-flow-strip-types": "^7.0.0",
     "@babel/preset-env": "^7.0.0",
     "@babel/preset-flow": "^7.0.0",