[Input] Fix for ProjectWideActionsExample scene (avoid dereferencing deleted GameObject)

[AlexTyrer]

Description

Fix for ProjectWideActionsExample scene (avoid dereferencing deleted GameObject)

o Ensure InputAction callbacks are removed on MonoBehaviour destruction o The callbacks were dereferencing the script's cube GameObject (after the scene was unloaded)

Changes made

Moved the attack/cancel callbacks to explicit functions so they can be removed by name in the script's OnDestroy() function.

Checklist

Before review:

• [ ] Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• [ ] Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• [ ] Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• [ ] Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.