npc.php: remove $underAttack variable
npc.php: remove $underAttack variable

Removing this tracking variable doesn't change the logic of the
script. The `$player->isUnderAttack()` achieves the same purpose,
so long as we're not calling `$player->removeUnderAttack()` in any
of the page processing.

hemberger/smr,hemberger/smr,hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr,smrealms/smr