Remove obsolete pixma macOS workaround

Without this change, pixma scanning is currently broken (tested with Canon MP495). The stated reason for the workaround (libusb not supporting timeouts for interrupts) hasn't been true since 2010:
67d9ef7b68

backend/pixma/pixma_io_sanei.c

@@ -62,13 +62,6 @@
 # define UNUSED(v)
 #endif
 
-/* MAC OS X does not support timeouts in darwin/libusb interrupt reads
- * This is a very basic turnaround for MAC OS X
- * Button scan will not work with this wrapper */
-#ifdef __APPLE__
-# define sanei_usb_read_int sanei_usb_read_bulk
-#endif
-
 
 struct pixma_io_t
 {