[11.x] Add an option to remove the original environment file after encrypting

[riasvdv]

When managing environment files for multiple environments, it's useful to be able to remove the original file when encrypting to prevent outdated environment files from sticking around.

This adds an extra --prune option to the php artisan env:encrypt command that instantly removes the original environment file after encrypting.

php artisan env:encrypt --env=production --prune

Similar tools like git-secret remove the original file by default.

[taylorotwell]

Can we align the option name with other similar options? I think we use prune on migration squashing.

[riasvdv]

@taylorotwell renamed!