Bug 21073: (QA follow-up) Simplify logic

Bug 21073: (QA follow-up) Simplify logic

This patch simplifies the logic inside GetPlugins so:
- It uses Koha::Plugins::Methods instead of plain SQL
- It doesn't do more DB calls than needed, by filtering on method in the
  initial query to Koha::Plugins::Methods.

It also relies on the (newly introduced) ->is_enabled method in
Koha::Plugins::Base, for better readability.

To test:
- Run the tests and notice no behaviour changes are introduced.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>