distmaker - Finish migrating bower.json => composer.json

[totten]

Overview

This is a follow-up to #15044 - since we don't have/need bower.json, it doesn't make sense for distmaker to call bower install.

Before

• distmaker tries to call bower install and publish bower.json

After

• distmaker omits bower install and bower.json

[civibot]

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[eileenmcnaughton]

I think the tarballs get pretty tested during release & rc process so happy to merge this & allow the work to progress