Stop showing priorities for other provider types

This isn't used and showing priorities when we only have a single
provider or where they have no effect is unnecessarily confusing.
Removing the form makes it clearer that there's only one way to
adjust priorities for domestic SMS providers.

If we add another email or international SMS provider in future,
we would need to rewrite the form here anyway as the priorities
need to be adjusted in tandem, not individually.

app/main/forms.py

@@ -1740,12 +1740,6 @@ class EstimateUsageForm(StripWhitespaceForm):
         return super().validate(*args, **kwargs)
 
 
-class AdminProviderForm(StripWhitespaceForm):
-    priority = GovukIntegerField(
-        'Priority', [validators.NumberRange(min=1, max=100, message="Must be between 1 and 100")]
-    )
-
-
 class AdminProviderRatioForm(StripWhitespaceForm):
 
     ratio = GovukRadiosField(choices=[