How does Pip's discussion with Jaggers disabuse him of the notion that Miss Havisham has been his patron in Great Expectations?
I do not know which discussion between Pip and Jaggers you are talking about here so I do not know how Pip would have been disabused.
To me, Pip learns who his patron was in Chapter XXXIX. In that chapter, the convict, Magwitch, comes to Pips apartments and has a chat with him. He tells Pip that he has been Pip's benefactor. Pip asks him if Miss Havisham had anything to do with it and Magwitch says she did not.
You can see from the following quote that Pip did not know who his patron was before this conversation:
"Well, you see it wos me, and single-handed. Never a soul in it but my own self and Mr. Jaggers."
"Was there no one else?" I asked.
"No," said he, with a glance of surprise: "who else should there be?