Remove JSON from default asset types (#593)

Summary:
**Summary**

JSON files are currently considered both source and assets so they are duplicated in the js bundle and in the assets copied in the final app. I think this is a better default and if someone relies on json files being also included in the bundle they can add it back in their metro config.

Fixes #487

**Test plan**

Test that json files are no longer present in my app assets and the app still works.

Pull Request resolved: #593

Reviewed By: MichaReiser

Differential Revision: D24904641

Pulled By: cpojer

fbshipit-source-id: 934b31fc022b3c32f57c88d17eaee22a2808728a

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -31,7 +31,6 @@ Object {
       "mp3",
       "wav",
       "html",
-      "json",
       "pdf",
       "yaml",
       "yml",
@@ -169,7 +168,6 @@ Object {
       "mp3",
       "wav",
       "html",
-      "json",
       "pdf",
       "yaml",
       "yml",
@@ -307,7 +305,6 @@ Object {
       "mp3",
       "wav",
       "html",
-      "json",
       "pdf",
       "yaml",
       "yml",
@@ -445,7 +442,6 @@ Object {
       "mp3",
       "wav",
       "html",
-      "json",
       "pdf",
       "yaml",
       "yml",

packages/metro-config/src/defaults/defaults.js

@@ -38,7 +38,6 @@ exports.assetExts = [
   'wav',
   // Document formats
   'html',
-  'json',
   'pdf',
   'yaml',
   'yml',