All of the mathematicians I mentioned believed philosophy was relevant to their mathematical work, and that period of work on the foundations of mathematics was accompanied by extensive discussion of the work of Frege, Russell, Wittgenstein, etc. Even if we pretend philosophy never involves rigorous proofs, mathematical theorems do not spring out of thin air, and saying "anything that isn't a formal, rigorous proof is useless to mathematics" is like saying "anything that isn't a finished house is useless to the process of building a house".
https://arxiv.org/pdf/math/9404236.pdf discusses this in more detail.
Here's a paper by Tarski, widely cited by both mathematicians and philosophers and containing both formal and informal reasoning: http://www.thatmarcusfamily.org/philosophy/Course_Websites/R... I don't know how one could "remove the philosophy" from this work without making it far less useful to mathematicians. The entire reason the T-schema is used in model theory is because of Tarski's philosophical argument that it provides a meaningful definition of truth.