I wouldn't think so. "Sincere" is from Latin "sincerus" meaning "pure, whole, natural". This is an adjective with a whole declension or set of different forms for different grammatical persons, genders, and numbers ("sincerus, sincera, sincerum" and so on, up to a couple dozen forms!). If this word had indeed been formed from a combination of Lat "sine" (without") and "cera" ("wax"), it is more likely that it would have remained simply as an adverbial expression "sine cera", or simply as the adverb (Lat) "sincere", than that it would have yielded the whole complete set of forms needed for declension.

At least, that's my impression.