npm run clean-build is broken

[samuelallan72]

It looks like webpack-cli build no longer provides a --clean option:

$ npm run clean-build

> edx-ora2@6.16.3 clean-build
> fedx-scripts webpack --clean

Running with resolved config:
/mnt/edx-ora2/webpack.prod.config.js

[webpack-cli] Error: Unknown option '--clean'
[webpack-cli] Run 'webpack --help' to see available commands and options

And according to these lines, cleaning the dist directory is always done automatically as part of the build:

edx-ora2/webpack.prod.config.js

Lines 100 to 101 in 3e4bf88

	// Cleans the dist directory before each build
	new CleanWebpackPlugin(),

So the clean-build script can probably be completely removed.