fix: Revert "feat: Add security model signals for User, Role, and Group CRUD operations (#2432)"

[dpgaspar]

Summary

• Reverts commit ca5bdfd (feat: Add security model signals for User, Role, and Group CRUD operations #2432)
• Removes security model signals (pre/post commit signals for User, Role, and Group CRUD operations)
• Removes flask_appbuilder/security/events.py, flask_appbuilder/security/signals.py, and tests/test_security_signals.py
• Reverts signal-related changes from flask_appbuilder/security/sqla/manager.py and flask_appbuilder/const.py

Test plan

• Verify existing tests pass without the signals code
• Confirm no regressions in User, Role, and Group CRUD operations

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