Moves the bulk of shield code from the riot subtype into the core shield. (#71066)

## About The Pull Request
I wanted to "rework" the riot shield security has but most other shields
are subtypes of it, making them all inherit those features.
So I made this PR first to make my job easier and fix some odd bugs like
fixing wooden bucklers by slapping them with titanium sheets...

There *shouldn't* be any other big change outside of the titanium
healing and baton bashing they all inherited but I have been staring at
shield code and worked on another branch before deciding to split the
code improvement/fixes from the features so I might have missed
something.

And while there, kill some single letter vars, remove unnecessary lines,
etc.
## Why It's Good For The Game
Fixing a wooden shield by slapping it with titanium is weird... so is
fixing a glass+iron shield but that is a bigger balance change.

And well, easier for future shield changes to not inherit weird
behaviors from the riot shield.
## Changelog
🆑 Guillaume Prata
fix: You can no longer repair wooden bucklers and roman shields by
slapping them with a titanium sheet.
/🆑

code/modules/mob_spawn/corpses/mining_corpses.dm

@@ -192,7 +192,7 @@
 	if(prob(30))
 		back = /obj/item/nullrod/scythe/talking
 	else
-		back = /obj/item/shield/riot/buckler
+		back = /obj/item/shield/buckler
 		belt = /obj/item/nullrod/claymore
 
 /datum/outfit/consumed_shadowperson