Removed several deprecated components

config/autoload/app_options.global.php

@@ -2,14 +2,11 @@
 
 declare(strict_types=1);
 
-use function Shlinkio\Shlink\Common\env;
-
 return [
 
     'app_options' => [
         'name' => 'Shlink',
         'version' => '%SHLINK_VERSION%',
-        'secret_key' => env('SECRET_KEY', ''),
         'disable_track_param' => null,
     ],
 

config/autoload/error-handler.global.php

@@ -15,10 +15,6 @@ return [
         ],
     ],
 
-    'backwards_compatible_problem_details' => [
-        'json_flags' => JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE | JSON_PRESERVE_ZERO_FRACTION,
-    ],
-
     'error_handler' => [
         'listeners' => [Logger\ErrorLogger::class],
     ],

config/autoload/middleware-pipeline.global.php

@@ -21,7 +21,6 @@ return [
             'path' => '/rest',
             'middleware' => [
                 Rest\Middleware\CrossDomainMiddleware::class,
-                Rest\Middleware\BackwardsCompatibleProblemDetailsMiddleware::class,
                 ProblemDetails\ProblemDetailsMiddleware::class,
             ],
         ],
@@ -35,7 +34,6 @@ return [
             'path' => '/rest',
             'middleware' => [
                 Rest\Middleware\PathVersionMiddleware::class,
-                Rest\Middleware\ShortUrl\ShortCodePathMiddleware::class,
             ],
         ],