Fix RIVAL1's first Pokémon not having a held item

RIVAL1 in this generation is hardcoded so that his first Pokémon never utilizes the held item it gets. This can easily confound new hackers, but is simple to fix.

Go to engine/battle/core.asm and delete the following lines:

InitEnemyTrainer:
	ld [wTrainerClass], a
	farcall StubbedTrainerRankings_TrainerBattles
	xor a
	ld [wTempEnemyMonSpecies], a
	callfar GetTrainerAttributes
	callfar ReadTrainerParty

-	; RIVAL1's first mon has no held item
-	ld a, [wTrainerClass]
-	cp RIVAL1
-	jr nz, .ok
-	xor a
-	ld [wOTPartyMon1Item], a
-
-.ok
	ld de, vTiles2
	callfar GetTrainerPic
	...

This block checks if the opposing trainer is RIVAL1, and if so, changes the value of their first Pokémon's held item to 0, meaning no item. Why exactly Game Freak did this when RIVAL1's parties don't actually have any held items is unclear, but we can just delete the code to let him use held items like any other trainer.