Remove migrations from autoload classmap

[garygreen]

The list of migrations in your app can grow massively and these currently automatically get included in your composer classmap. Migrations are intended to be run as a one-time thing and are already automatically required so this PR avoids unnecessarily adding the migrations to your composer file.

[KKSzymanowski]

Is the autoload dump still required after creating a new migration?
I'm referring to this: https://github.com/laravel/framework/blob/7.x/src/Illuminate/Database/Console/Migrations/MigrateMakeCommand.php#L96