Cannot purge everything

Problem/Motivation

The AkamaiPurger claims that it can do an everything purge.

* types = {"path", "url", "everything"},

But if you actually try to do an everything purge, it throws an error. I don't see how it could do an everything purge without having a CP Code stored in the configuration that it would use to do the purge. And there's no sign of that.

Steps to reproduce

Run drush pinv everything

drush pinv everything

 !                                                                                                                      
 ! [CAUTION] Invalidating everything will mass-clear potentially thousands of pages, which could temporarily make your  
 !           site really slow as external caches will have to warm up again.                                            
 !                                                                                                                      

 Are you really sure? (yes/no) [yes]:
 > yes

 [error]  httpStatus: 400 
detail: 'objects' attribute cannot be missing or empty 
supportId: <redacted>
title: invalid attribute value 
describedBy: https://developer.akamai.com/api/core_features/fast_purge/v3.html#httpcodes 

Proposed resolution

Unless there are plans to actually add everything support to clear caches, and until that happens, we should just remove the "everything" from the plugin definition. I think that should A) make it not show up in the purge configuration page that it can do that and B) not try to fire off an API request when running an everything purge.

Remaining tasks

User interface changes

API changes

Data model changes