[guix] Remove unused geniso override

Summary: We use xorrisofs instead.

Test Plan:

./contrib/guix/guix-clean && HOSTS="x86_64-apple-darwin" ./contrib/guix/guix-build

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15649