remove experimental.generateMissingPreprocessorSourcemaps

[dominikg]

Describe the problem

it needs an extra dependency and is very slow for larger codebases.

Describe the proposed solution

remove it

Alternatives considered

Alternatively we need to make sure preprocessors always generate sourcemaps.

With the future import queries support it will be possible to showcase missing sourcemaps in a better way, use that to help users prod preprocessor authors for improved support.

Importance

would make my life easier