Refactor: remove TargetPointer::getFirstAsFixedTarget

[Susucre]

A single card is using it [[Giant Oyster]]. I don't see a reason for the method. It was actually used by a bugged version of Narci, Fabled Singer, cleaned in 5016a57.

[github-actions]

Giant Oyster - (Gatherer) (Scryfall) (EDHREC)

{2}{U}{U}
Creature — Oyster
0/3
You may choose not to untap Giant Oyster during your untap step.
{T}: For as long as Giant Oyster remains tapped, target tapped creature doesn't untap during its controller's untap step, and at the beginning of each of your draw steps, put a -1/-1 counter on that creature. When Giant Oyster leaves the battlefield or becomes untapped, remove all -1/-1 counters from the creature.