pbio/drv/usb_stm32: remove subscribe timeout

Remove the transmit_timer that was monitoring whether the host was
reading data or not. In practice, this was causing false positives
when the host was just slow to read data, which caused the connection
to effectively drop.

Author: dlech

lib/pbio/drv/usb/usb_stm32.c

@@ -399,7 +399,6 @@ PROCESS_THREAD(pbdrv_usb_process, ev, data) {
     static bool bcd_busy;
     static pbio_pybricks_error_t result;
     static struct etimer status_timer;
-    static struct etimer transmit_timer;
     static uint32_t prev_status_flags = ~0;
     static uint32_t new_status_flags;
 
@@ -474,13 +473,6 @@ PROCESS_THREAD(pbdrv_usb_process, ev, data) {
         }
 
         if (transmitting) {
-            if (etimer_expired(&transmit_timer)) {
-                // Transmission has taken too long, so reset the state to allow
-                // new transmissions. This can happen if the host stops reading
-                // data for some reason.
-                pbdrv_usb_stm32_reset_tx_state();
-            }
-
             continue;
         }
 
@@ -511,12 +503,6 @@ PROCESS_THREAD(pbdrv_usb_process, ev, data) {
                 USBD_Pybricks_TransmitPacket(&husbd, usb_stdout_buf, usb_stdout_sz);
             }
         }
-
-        if (transmitting) {
-            // If the FIFO isn't emptied quickly, then there probably isn't an
-            // app anymore. This timer is used to detect such a condition.
-            etimer_set(&transmit_timer, 50);
-        }
     }
 
     PROCESS_END();