Can one omit the relative pronouns i/ya in Quenya?

Sometimes, we can omit the relative pronoun "that" without changing the sentence's meaning in English. For example: "It was the best film (that) I've ever seen.", where the relative pronoun "that" can be omitted. Can we do that in Quenya, which means omit the word "i" or "ya"?

Very probably not. While this phenomenon isn’t restricted to just English, it is not common enough to suspect it by default and as far as I know we have no reason to suspect it in Quenya (i. e. examples, descriptions....). There is an abudance of participles though that can usually archive similar effects:

• Eldar coitaila Veleriadesse Valariandesse ohta carner Moricottonna “The Elves living in Beleriand made war against Morgoth”
• Arcastar estane macil mahtaina ló Túrin Ñuranga “Arcastar called the sword wielded by Túrin Gurthang”

(I'd definitely recommend Veleriadesse > Valariandesse, but agreed 100% otherwise)