Nobody in particular, it is what is called collective image in literature. Feynman's attitude towards philosophy is expressed by another of his quotes, "philosophy of science is about as useful to scientists as ornithology is to birds", or "we get into that paralysis of thought that comes to philosophers… one saying to the other: you don’t know what you are talking about! The second one says: what do you mean by ‘talking’? What do you mean by ‘you’? What do you mean by ‘know’?”". It seems rather unlikely that he read much of them. The irony is that Feynman himself philosophized about science quite a lot.
"A philosopher once said" is a figure of speech Feynman uses (similar to "a wise man once said", but with negative connotation) to express what he imagines philosophers might have said. The OP quote is from Character of Physical Law, Chapter 6. In Lectures on Physics, volume I, chapter 2 the quote is attributed to "philosophers" in the plural, and "some or other":
"This is a horrible thing; in fact, philosophers have said before that one of the fundamental requisites of science is that whenever you set up the same conditions, the same thing must happen. This is simply not true, it is not a fundamental condition of science. The fact is that the same thing does not happen, that we can find only an average, statistically, as to what happens. Nevertheless, science has not completely collapsed. Philosophers, incidentally, say a great deal about what is absolutely necessary for science, and it is always, so far as one can see, rather naive, and probably wrong. For example, some philosopher or other said it is fundamental to the scientific effort that if an experiment is performed in, say, Stockholm, and then the same experiment is done in, say, Quito, the same results must occur. That is quite false. It is not necessary that science do that; it may be a fact of experience, but it is not necessary."
In fairness, as the context of quantum mechanics makes clear, Feynman refers to the popularity of causal determinism among philosophers. It goes as far back as Democritus and Stoics, and was supported more recently by Hobbes, Spinoza, Leibniz, Kant, etc. Kant comes to mind in particular because the language of causality, etc., as "conditions of possibility of knowledge" was his signature way of justifying science. Of course, a great deal of scientists, like Laplace and late 19th century Laplacian determinists, were just as culpable, as are many contemporary scientists.
Another well known quote of Feynman's is:"A poet once said, "The whole universe is in a glass of wine"". There was no such poet either, see Do Bianchi's post:
"I believe that the imagery comes from a “scientific letter” by Italian philosopher Lorenzo Magalotti (1637-1712) who cites Galileo’s [attributed] maxim, wine is a compound [mixture] of moisture [humor] and light (il vino è un composto di umore e di luce)... This celebrated observation of the physical world was transmitted anecdotally by Galileo’s student Raffaello Magiotti (1597-1656), who is quoted by Magalotti in the letter."