Remove more traces of alpha flavour (#51)

The alpha flavour is still unavailable as a consequence of work
left to do with respect to the following issue:

commons-app/apps-android-commons#4487

android/Volunteers-welcome!.md

@@ -13,20 +13,14 @@ It is also recommended that you actually try the app and upload pictures with th
 
 # Testers
 
-Use the beta version, or even the alpha version, and report bugs to the [issue tracker](https://github.com/commons-app/apps-android-commons/issues/).
+Use the beta version and report bugs to the [issue tracker](https://github.com/commons-app/apps-android-commons/issues/).
 
 Choose the beta version if:
 
 - You want to test only relatively stable versions of the app.
 - You consider that uploading your pictures is more important than finding the app's bugs.
 - You don't want to receive many app updates, because you don't have access to unlimited Internet.
 
-Choose the alpha version if:
-
-- You want to test the latest cutting edge.
-- You love finding bugs, and don't mind if some features are unusable sometimes.
-- You don't mind receiving app updates several times per day.
-
 ## How to install the beta
 
 The beta version might have bugs, your role is to find them all so that we can make a bug-free stable release :-)