"I had decided not to go."
Translation:Yo había decidido no ir.
The "que" is not necessary because the infinitive "ir" (to go) is acting as a noun substitute because it is the direct object of the Spanish compound verb "había decidido. Because infinitives are verbs, they can get negated by the adverb "no," in both Spanish and English.
That would be "I had decided not to leave." That's a valid interpretation of this sentence, but it only works if the emphasis is on the speaker leaving their current place and not on where they'd be going. In the latter case the reflexive would be inappropriate. So if the speaker meant "I had decided not to go (to the party)," it would be "Había decidido no ir," or "Había decidido no ir a la fiesta."