hub75: don't enable an unrelated IRQ

DREQs are not IRQs, `DREQ_PIO0_TXn` happens to align with `TIMER_IRQ_n` so this enabled a timer IRQ.

drivers/hub75/hub75.cpp

@@ -146,7 +146,6 @@ void Hub75::start(irq_handler_t handler) {
 
         dma_channel_set_irq0_enabled(dma_channel, true);
 
-        irq_set_enabled(pio_get_dreq(pio, sm_data, true), true);
         irq_set_enabled(DMA_IRQ_0, true);
 
         row = 0;
@@ -275,4 +274,4 @@ void Hub75::update(PicoGraphics *graphics) {
         }
     }
 }
-}
+}