[Firefox] Stop importing default_preferences.js as a module and include it instead

*Inspired by this thread, started by @nnethercote on dev.platform: https://groups.google.com/forum/#!topic/mozilla.dev.platform/sAuhWQOAWic.*

This PR makes use of the preprocessor to include the file default_preferences.js, instead of loading it as a module in the `MOZCENTRAL`/`FIREFOX` builds.
 
The only way to test this is unfortunately to apply the patch, run `node make {mozcentral, firefox}`, and then check that `DEFAULT_PREFERENCES` is included properly in `initializeDefaultPreferences` (i.e. lines https://github.com/mozilla/pdf.js/blob/master/web/default_preferences.js#L23-L27 should appear) in the following files:
* `MOZCENTRAL` - build/mozcentral/browser/extensions/pdfjs/content/PdfJs.jsm.
* `FIREFOX` - build/firefox/bootstrap.js.
 
Note that the file default_preferences.js should not be present in the build directories, since it's no longer necessary.