module: drop unused module_writable_address()

module_writable_address() is unused and can be removed.

Signed-off-by: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250126074733.1384926-9-rppt@kernel.org