Remove use of projects/browser/run_scripts

In projects/browser/build we are creating a script (in "$scripts_dir/create-$PKG_DIR"), and use run_scripts to run it.

This was added in dfa0cc46 and was useful to generate bundles for the different locales in parallel. However since we now have one bundle for all locales, it's not useful anymore, and we can simplify the build script by not doing that.

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