Projectile ref clears (#5164)

# About the pull request

Forgot to clear a reference I set in
#4986 and cleared some others
I saw.

Not familiar enough with the Destroy/QDEL pipeline to know what else, if
anything, should be done with `weapon_cause_data` and `bullet_traits`
since they hold references themselves.

<!-- Remove this text and explain what the purpose of your PR is.

Mention if you have tested your changes. If you changed a map, make sure
you used the mapmerge tool.
If this is an Issue Correction, you can type "Fixes Issue #169420" to
link the PR to the corresponding Issue number #169420.

Remember: something that is self-evident to you might not be to others.
Explain your rationale fully, even if you feel it goes without saying.
-->

# Explain why it's good for the game

I will clean up after myself.
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
No player-facing changes.

code/modules/projectiles/projectile.dm

@@ -96,7 +96,10 @@
 	starting = null
 	permutated = null
 	path = null
+	vis_source = null
+	process_start_turf = null
 	weapon_cause_data = null
+	bullet_traits = null
 	firer = null
 	QDEL_NULL(bound_beam)
 	SSprojectiles.stop_projectile(src)