[2.x] chore(flags): dispatch model events when flags are deleted

[DavideIadeluca]

Fixes #0000
No eloquent model events being dispatched when a flag gets deleted

Changes proposed in this pull request:

• Change how flags are deleted. With the new approach model events are fired after each delete, allowing Observers to hook into this.
• This in turn also allows the Audit Integration to loose some complexity with the macro

Reviewers should focus on:

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)