Removed prg() plugin, as it is now in zend-mvc-plugin-prg

This patch removes `Zend\Mvc\Controller\Plugin\PostRedirectGet`, as it is now
available via the [zend-mvc-plugin-prg](https://zendframework.github.io/zend-mvc-plugin-prg)
package.

Author: weierophinney

doc/book/migration/to-v3-0.md

@@ -77,3 +77,24 @@ ensure console tooling continues to work.
 > recommend migrating your code to use standalone tools such as
 > [zf-console](https://github.com/zfcampus/zf-console) or
 > [Aura.Cli](https://github.com/auraphp/Aura.Cli).
+
+## Plugins
+
+The following plugins have been removed from the main zend-mvc repository, and
+into their own standalone repositories. In all cases, please be sure to install
+the [component installer as detailed above](#dependency-reduction) before
+installing the plugins, to automate injection into your application
+configuration.
+
+### prg()
+
+The `prg()` plugin is now provided via the
+[zend-mvc-plugin-prg](https://github.com/zendframework/zend-mvc-plugin-prg)
+component.
+
+```bash
+$ composer require zendframework/zend-mvc-plugin-prg
+```
+
+`Zend\Mvc\Controller\Plugin\PostRedirectGet` becomes
+`Zend\Mvc\Plugin\Prg\PostRedirectGet`. However, it is still mapped as `prg()`.

src/Controller/Plugin/PostRedirectGet.php

@@ -52,10 +52,6 @@ class PluginManager extends AbstractPluginManager
         'layout'                      => Plugin\Layout::class,
         'Params'                      => Plugin\Params::class,
         'params'                      => Plugin\Params::class,
-        'PostRedirectGet'             => Plugin\PostRedirectGet::class,
-        'postRedirectGet'             => Plugin\PostRedirectGet::class,
-        'postredirectget'             => Plugin\PostRedirectGet::class,
-        'prg'                         => Plugin\PostRedirectGet::class,
         'Redirect'                    => Plugin\Redirect::class,
         'redirect'                    => Plugin\Redirect::class,
         'Url'                         => Plugin\Url::class,
@@ -76,7 +72,6 @@ class PluginManager extends AbstractPluginManager
         Plugin\FlashMessenger::class              => InvokableFactory::class,
         Plugin\Layout::class                      => InvokableFactory::class,
         Plugin\Params::class                      => InvokableFactory::class,
-        Plugin\PostRedirectGet::class             => InvokableFactory::class,
         Plugin\Redirect::class                    => InvokableFactory::class,
         Plugin\Url::class                         => InvokableFactory::class,
         Plugin\CreateHttpNotFoundModel::class     => InvokableFactory::class,
@@ -90,7 +85,6 @@ class PluginManager extends AbstractPluginManager
         'zendmvccontrollerpluginflashmessenger'              => InvokableFactory::class,
         'zendmvccontrollerpluginlayout'                      => InvokableFactory::class,
         'zendmvccontrollerpluginparams'                      => InvokableFactory::class,
-        'zendmvccontrollerpluginpostredirectget'             => InvokableFactory::class,
         'zendmvccontrollerpluginredirect'                    => InvokableFactory::class,
         'zendmvccontrollerpluginurl'                         => InvokableFactory::class,
         'zendmvccontrollerplugincreatehttpnotfoundmodel'     => InvokableFactory::class,