stm32/qstrdefsport.h: Remove unused qstrs and make USB ones conditional.
qstrs in this file are always included in all builds, even if not used
anywhere.  So remove those that are never needed, and make USB names
conditional on having USB enabled.
// qstrs specific to this port
-Q(boot.py)
-Q(main.py)
// Entries for sys.path
Q(/flash)
Q(/flash/lib)
Q(/sd)
Q(/sd/lib)
+
+// For uos.sep
+Q(/)
+
+#if MICROPY_HW_ENABLE_USB
// for usb modes
Q(MSC+HID)
Q(VCP+MSC)
Q(VCP+HID)
-Q(CDC+MSC)
-Q(CDC+HID)
-Q(/)
-
-
-// The following qstrings not referenced from anywhere in the sources
-Q(CDC)
-Q(flash)
+#endif
