How do you say "All death is certain" in latin?

Actually, no, you shouldn't omit the word "omnes" in this case, as otherwise your sentence would simply say "Death is certain", which is not really what you want. If you're looking for something to omit, you can always cut out the word "est" ("is"), which can usually be omitted in Latin.

As for the rest of the translation, there's a slight grammatical problem here. The word "all", by its very nature, is plural. There IS a singular form of the word "omnes", but it's actually translated as "every" or "every single". So you could say Omnis mors certa (est), but this means "every single death is certain". If you strictly want to say "all" (i.e., plural), that's: Omnes mortes certae (sunt) ("All deaths are certain"). Note the ending of "certa/certae" in both examples.

Omit the 'omnes' part. Latin loves shortness in pithy sayings. But await answers from Latin experts - Google Translate is notoriously bad for Latin.