Fix GENERATE_SOURCEMAP and revert splitChunk

[Gnito]

Currently released as 6.0.1-RC

2 fixes:

Remove vendor code chunk

splitChunks was reintroduced prematurely.
CRA 5 is discussing reintroducing it, but the previous (CRA4) style does not work with 'async' marked thunk functions.

I.e. this was removed:

      // Sharetribe custom: create vendor chunk
      // CRA v5, removed this splitChunks feature, but they are planning to reintroduce it.
      // https://github.com/facebook/create-react-app/pull/11763
      splitChunks: isEnvDevelopment
        ? undefined
        : {
            chunks: 'all',
          },      

Error when using env variable: GENERATE_SOURCEMAP=false

GENERATE_SOURCEMAP=false caused an error since it modified the structure of webpack.config.js