Do not use external pullups on CoreBoard F401RC

This board also has internal pullups, so the external one is not
actually needed (and will even violate the USB spec when both are used
together). This commit disabled the external pullups, but leaves the
defines in comments, as future documentation.

See also stm32duino#1029.

Author: matthijskooijman

variants/Generic_F401Rx/variant.h

@@ -127,8 +127,11 @@ extern "C" {
 
 #ifdef ARDUINO_CoreBoard_F401RC
 // USB, pull this pin low to enable the USB attach pullup
-#define USBD_ATTACH_PIN         PD2
-#define USBD_ATTACH_LEVEL       LOW
+// It is documented here, but not actually used, since there are also
+// internal pullups which are automatically used and using both would
+// violate the USB specification for pullup strength.
+//#define USBD_ATTACH_PIN         PD2
+//#define USBD_ATTACH_LEVEL       LOW
 #endif
 
 #ifdef __cplusplus