[devops] Remove the default 'xcodeChannel' value for the API diff.

The consumers of the API stage already passes a value, so a default is not needed.

This reduces the number of places we have to change every time we switch between stable and beta.

tools/devops/automation/templates/api-diff-stage.yml

@@ -25,7 +25,6 @@ parameters:
 
 - name: xcodeChannel
   type: string
-  default: Beta
 
 - name: macOSName
   type: string