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
