Remove vestigal dualcore feature gates

Were needed before [stm32-rs/285](stm32-rs/stm32-rs#285)

Author: richardeoin

src/prelude.rs

@@ -1,10 +1,8 @@
 //! Prelude
 pub use embedded_hal::prelude::*;
 
-#[cfg(all(feature = "device-selected", feature = "singlecore"))]
 pub use crate::adc::AdcExt as _stm32h7xx_hal_adc_AdcExt;
 pub use crate::delay::DelayExt as _stm32h7xx_hal_delay_DelayExt;
-#[cfg(all(feature = "device-selected", feature = "singlecore"))]
 pub use crate::flash::FlashExt as _stm32h7xx_hal_flash_FlashExt;
 pub use crate::gpio::GpioExt as _stm32h7xx_hal_gpio_GpioExt;
 pub use crate::i2c::I2cExt as _stm32h7xx_hal_i2c_I2cExt;