refactor(types)!: remove handler and update docs (#17358)

types/handler.go

@@ -1,8 +1,5 @@
 package types
 
-// Handler defines the core of the state transition function of an application.
-type Handler func(ctx Context, msg Msg) (*Result, error)
-
 // AnteHandler authenticates transactions, before their internal messages are handled.
 // If newCtx.IsZero(), ctx is used instead.
 type AnteHandler func(ctx Context, tx Tx, simulate bool) (newCtx Context, err error)