Drop the TestOptionalAccount adapter

It's included in the gem lib which is best to avoid and we can infer
that it works account optional adapters.

lib/kamal/secrets/adapters/test_optional_account.rb

@@ -1,5 +0,0 @@
-class Kamal::Secrets::Adapters::TestOptionalAccount < Kamal::Secrets::Adapters::Test
-  def requires_account?
-    false
-  end
-end

test/cli/secrets_test.rb

@@ -13,12 +13,6 @@ class CliSecretsTest < CliTestCase
       run_command("fetch", "foo", "bar", "baz", "--adapter", "test")
   end
 
-  test "fetch without required --account" do
-    assert_equal \
-      "\\{\\\"foo\\\":\\\"oof\\\",\\\"bar\\\":\\\"rab\\\",\\\"baz\\\":\\\"zab\\\"\\}",
-      run_command("fetch", "foo", "bar", "baz", "--adapter", "test_optional_account")
-  end
-
   test "extract" do
     assert_equal "oof", run_command("extract", "foo", "{\"foo\":\"oof\", \"bar\":\"rab\", \"baz\":\"zab\"}")
   end