It was never complete, even for F1 family code, and went on to be even less complete for f0 and f3. The usefulness of a library function to check for both the irq being enabled _and_ the status flag is highly questionable, and caused known user confusion. The existing, much simpler, and fully functional usart_get_flag() is a good replacement in almost all sane use cases. Fixes https://github.com/libopencm3/libopencm3/issues/734