Remove create_button

#### Description
Sources/Theme.php createButton()

A note exists that it was deprecated in 2.1.
In 2.0 we added the ability to use a custom create button logic:
```
! Allow themes to use "template_create_button" to create their own buttons (Subs.php) [Bug 1878]
```

I searched the entire customize site, could not find not one reference to this.

I think we are safe to remove it.