Remove pnpm support from build scripts

Actually just figured we removed it from our custom child themes that are using bs_bootstrap theme. However I just noticed we still have pnpm-lock.yaml in bs_base and it is making problems if you used `npm install` and then `npm run build` which will try to use pnpm first if available and throw all kind of errors because of previous `npm install`.

Also npm feels much faster now and there are no benefits using pnpm in ddev enviroment, so let's remove pnpm support now and not latter. You can still use it in your custom themes if needed, just edit your child themes package.json and bring back pnpm there in build scripts.

Merged.

Automatically closed - issue fixed for 2 weeks with no activity.