Bug 20438: Allow uninstalling plugins not implementing the 'uninstall' method

Bug 20438: Allow uninstalling plugins not implementing the 'uninstall' method

The plugins handling code expects plugins implementing an 'uninstall' method for
cleanup purposes. It executes this method, and then moves on with removing the
configuration entries in the storage, and the plugin itself.

But the 'plugins-home.tt' template makes the tool display the 'Uninstall'
link in the dropdown ONLY when the plugin has the 'uninstall' method.

This patch fixes the issue.

To reproduce:
- Install the KitchenSink plugin [1]
=> SUCCESS: The 'actions' dropdown for KitchenSink shows an 'Uninstall' option
- Edit the code for the plugin:
  $ sudo vim /var/lib/koha/kohadev/plugins/Koha/Plugin/Com/ByWaterSolutions/KitchenSink.pm
- Remove the uninstall sub
- Restart everything:
  $ restart_all
- Reload the browser
=> FAIL: There's no 'Uninstall' option in the actions dropdown.
- Apply this patch
- Restart everything
  $ restart_all
- Reload the browser
=> SUCCESS: The 'actions' dropdown for KitchenSink shows an 'Uninstall' option
- Sign off :-D

[1] https://github.com/bywatersolutions/koha-plugin-kitchen-sink/releases

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>