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/stm32h7/src/bin/low_level_timer_api.rs

@@ -10,7 +10,6 @@ use embassy::executor::Spawner;
 use embassy::time::{Duration, Timer};
 use embassy::util::Unborrow;
 use embassy_hal_common::unborrow;
-use embassy_stm32::gpio::NoPin;
 use embassy_stm32::pwm::{pins::*, Channel, OutputCompareMode};
 use embassy_stm32::time::{Hertz, U32Ext};
 use embassy_stm32::timer::GeneralPurpose32bitInstance;
@@ -33,7 +32,7 @@ pub fn config() -> Config {
 async fn main(_spawner: Spawner, p: Peripherals) {
     info!("Hello World!");
 
-    let mut pwm = SimplePwm32::new(p.TIM5, p.PA0, NoPin, NoPin, NoPin, 10000.hz());
+    let mut pwm = SimplePwm32::new(p.TIM5, p.PA0, p.PA1, p.PA2, p.PA3, 10000.hz());
     let max = pwm.get_max_duty();
     pwm.enable(Channel::Ch1);