Stepper header without icons

#### Bug, feature request, or proposal:
Bug/feature request

#### What is the expected behavior?
Just display numbers in stepper headers

#### What is the current behavior?
Tries to display "create" in the icon after changing to another step

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-vpoj5j you can see it here

#### What is the use-case or motivation for changing an existing behavior?
Not everyone uses the icons

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 8.2.14
typescript 3.5.3

#### Is there anything else we should know?
I know I can override the icons, however I'm unsure about changing it to always using the 'number' state.
The UX of changing the stepper to these icons doesn't really appeal to me.

#### Additional info

doing this :

```

...

```

does not do it. the pencil icon continues to appear for opened states

Start with the StackBlitz reproduction and the mat-horizontal-stepper and matStepperIcon entry points shown in the issue. Determine how the stepper selects the edit and done icons, then verify that completed and active headers can display numbers without those icons.

索引モデルが issue の本文から書いたものです。