Get rid of some hacks in declarations file for webpack-4

[jeffin143]

What kind of change does this PR introduce?
Backport #10050 to webpack-4

Fix - #10217
Did you add tests for your changes?
NA

Does this PR introduce a breaking change?
None

What needs to be documented once your changes are merged?
Nothing

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

@jeffin143 Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@sokra Please review the new changes.

[jeffin143]

@sokra , need help with error :

declarations/WebpackOptions.d.ts need to be updated

[sokra]

@sokra , need help with error :

declarations/WebpackOptions.d.ts need to be updated

run yarn special-lint-fix

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[jeffin143]

@sokra should we still keep node-6 pipeline ?

[sokra]

Thanks