Bug 19735: (QA follow-up) Drop references to 'usage'

Bug 19735: (QA follow-up) Drop references to 'usage'

The 'usage' key was dropped as part of this migration to cpanfile but
it was still refered to by the installed. This patch simply removes
references to it. After considering how we might add it back in I
decided that as it stands it wasn't a well maintained list and should be
re-implimented in the future in a more consistent form if we miss it.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>