Remove the env check

The env check is not needded anymore as all the commands rely on the
env files having already been created remotely.

The only place the env is needed is when running `kamal env push` and
that will still raise an apropriate error.

test/configuration_test.rb

@@ -128,14 +128,6 @@ class ConfigurationTest < ActiveSupport::TestCase
     assert_equal "healthcheck-app", @config.healthcheck_service
   end
 
-  test "env with missing secret" do
-    assert_raises(KeyError) do
-      config = Kamal::Configuration.new(@deploy.tap { |c| c.merge!({
-        env: { "secret" => [ "PASSWORD" ] }
-      }) }).ensure_env_available
-    end
-  end
-
   test "valid config" do
     assert @config.valid?
     assert @config_with_roles.valid?