Even if Google did want to reduce effectiveness of ad blockers, doing that via removal of blocking webRequest API is a double-edged sword. It may push users to alternate browsers with more effective ad-blocking.

Besides, webRequest implementation in Chromium is a terrible collection of hacks on hacks. It is a good example how not to design or implement API. I will not be surprised if the removal of the API comes from a simple desire to remove that embarrassing code.