remove APP_TIMEZONE environment variable (#6536)

force the timezone to be defined in the config file, rather than deferring to an environment variable.

IMO having the timezone dependent on an environment variable adds an unnecessary amount of risk for your average user who may be unaware of the effects from changing this value.

some scenarios where this could cause issues:

- devs set this value to their local timezone on their local dev machines, and they are writing `Carbon:create()` commands or similar that now are working only under the assumption of that timezone value
- you have multiple production servers setup across the country and each has their own timezone value according to their location, but they all talk to a central database. now the database is loaded with mixed timezone variables

having an explicit value defined once for the application removes these risks.

reverts part of #6188

.env.example

@@ -2,7 +2,6 @@ APP_NAME=Laravel
 APP_ENV=local
 APP_KEY=
 APP_DEBUG=true
-APP_TIMEZONE=UTC
 APP_URL=http://localhost
 
 APP_LOCALE=en

config/app.php

@@ -65,7 +65,7 @@ return [
     |
     */
 
-    'timezone' => env('APP_TIMEZONE', 'UTC'),
+    'timezone' => 'UTC',
 
     /*
     |--------------------------------------------------------------------------