Assume tim2 in macro and remove clock setup in chip specific rcc init

Add temporary start_tim2() fn to Clock to assist macro in starting
embassy clock

embassy-stm32/src/rcc/h7/mod.rs

@@ -529,5 +529,4 @@ impl<'d> Rcc<'d> {
 }
 
 // TODO
-pub type SystemClock = ();
-pub unsafe fn init(_config: Config) -> SystemClock {}
+pub unsafe fn init(_config: Config) {}