stm32: Remove OptionalPin

The idea behind OptionalPin has a few problems:

- you need to impl the signal traits for NoPin which is a bit weird https://github.com/embassy-rs/embassy/blob/master/embassy-stm32/src/dcmi.rs#L413-L416
- you can pass any combination of set/unset pins, which needs checking at runtime  https://github.com/embassy-rs/embassy/blob/master/embassy-stm32/src/dcmi.rs#L130

The replacement is to do multiple `new` constructors for each combination of pins you want to take.

embassy-stm32/src/dac/mod.rs

@@ -3,27 +3,21 @@
 #[cfg_attr(dac_v1, path = "v1.rs")]
 #[cfg_attr(dac_v2, path = "v2.rs")]
 mod _version;
-use crate::gpio::NoPin;
 use crate::peripherals;
 pub use _version::*;
 
 pub(crate) mod sealed {
-    use crate::gpio::OptionalPin;
-
     pub trait Instance {
         fn regs() -> &'static crate::pac::dac::Dac;
     }
 
-    pub trait DacPin<T: Instance, const C: u8>: OptionalPin {}
+    pub trait DacPin<T: Instance, const C: u8>: crate::gpio::Pin {}
 }
 
 pub trait Instance: sealed::Instance + 'static {}
 
 pub trait DacPin<T: Instance, const C: u8>: sealed::DacPin<T, C> + 'static {}
 
-impl<T: Instance, const C: u8> DacPin<T, C> for NoPin {}
-impl<T: Instance, const C: u8> sealed::DacPin<T, C> for NoPin {}
-
 crate::pac::peripherals!(
     (dac, $inst:ident) => {
         impl crate::dac::sealed::Instance for peripherals::$inst {