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.

examples/stm32l4/src/bin/dac.rs

@@ -6,7 +6,6 @@
 mod example_common;
 
 use embassy_stm32::dac::{Channel, Dac, Value};
-use embassy_stm32::gpio::NoPin;
 use embassy_stm32::pac;
 use example_common::*;
 
@@ -22,7 +21,7 @@ fn main() -> ! {
 
     let p = embassy_stm32::init(Default::default());
 
-    let mut dac = Dac::new(p.DAC1, p.PA4, NoPin);
+    let mut dac = Dac::new_1ch(p.DAC1, p.PA4);
 
     loop {
         for v in 0..=255 {