stm32: flash: BSY bit has never been writable.

This has been copied around for years, but has never been a writable bit
on any target.

include/libopencm3/stm32/l4/flash.h

@@ -227,7 +227,6 @@ void flash_unlock(void);
 void flash_lock(void);
 void flash_clear_pgperr_flag(void);
 void flash_clear_eop_flag(void);
-void flash_clear_bsy_flag(void);
 void flash_wait_for_last_operation(void);
 void flash_dcache_enable(void);
 void flash_dcache_disable(void);