fix: delegate delete behavior to core

[maribethb]

Fixes #347

• Removes the custom delete shortcut as the behavior is fully covered by the one in core
• Removes most of the logic for the custom delete context menu item, and just adjusts the display text to add the shortcut key text
• Removes now-unused code
• Updates tests (note these tests are for behavior that is now controlled by core, but the tests in core are not working right now so per Rachel, have fixed them here with the understanding that they will be ported to core post-launch)