serial_bytes_available: don't double-count bytes on usb cdc

In 9.0.x, serial_bytes_available returned a bool and tud_cdc_available was
harmlessly checked twice for any characters.

When the routine was changed to return an int, the double checking led to
over-reporting the number of characters available. In code that would attempt
to read this many bytes from sys.stdin, this made the read call block since
only 1 byte was actually available.

This behavior came up in the discussion of adafruit#9393. I don't mark this bug as
closing that one, because that issue seems to be reporting multiple things that
this change would not address, such as delays in `sys.stdout.write()` or
problems seen while using webserial.

Author: jepler

supervisor/shared/serial.c

@@ -309,10 +309,6 @@ uint32_t serial_bytes_available(void) {
     }
     #endif
 
-    #if CIRCUITPY_USB_DEVICE
-    count += tud_cdc_available();
-    #endif
-
     // Board-specific serial input.
     count += board_serial_bytes_available();