libopkg: remove "extra_data" option

libopkg: remove "extra_data" option

The "extra_data" allowed to add an extra option behind a repository
definition. This was only ever used to either ignore a specific feed by
setting it to "__dummy__" or adding a sub-folder in the repository path,
as done for Debian repositories.

Instead of using "__dummy__" it is cleaner to "comment out" the specific
repository and Debian repository structure is nowhere used in the
OpenWrt ecosystem, therefore it seem save to remove "extra_data".

Signed-off-by: Paul Spooren <mail@aparcar.org>