remove webpack double error filtering hack

[sidoshi]

Changes verified by running this commands:

npm run create-react-app test-app
cd test-app
npm start

created a syntax error in App.js.
Error is not duplicated after removing the hack

[gaearon]

Can you check which version of webpack removed the need for the hack? Just to be sure.

[gaearon]

Note to self or @Timer: this will be a breaking change for react-dev-utils since we remove the workaround and technically people on older webpacks will have worse output.

[sidoshi]

v2.1.0-beta.11

[gaearon]

Interestingly, this hack may also be the reason for this disparity:

In this case I actually like the de-duplication and I wish it also happened in the terminal. We should probably investigate this a little bit more.

[gaearon]

Hey! I'm very sorry—I totally forgot about this PR and removed this part independently in #2113.

My observation in #1755 (comment) kept me from merging this, so I also did #2125 as a followup to avoid this problem.

I appreciate your help with this though!