Revert "Add <plugins> tag to config.xml, containing a list of plugins."

This reverts commit 7259921ceeededb6a280916c6898bc28a0a972ae.

We have decided that this approach is a mistake, and are pursuing an
alternative implementation. Code for that will follow.
Write plugin metadata (ID and version) into cordova_plugins.js

This new data is attached to the exported array as the key `metadata`.
It's an object whose keys are IDs and values are version strings.