Drop run_directory configuration option

We need to drop to be fixed so multiple applications put the config in
the same place.

test/commands/server_test.rb

@@ -12,10 +12,6 @@ class CommandsServerTest < ActiveSupport::TestCase
     assert_equal "mkdir -p .kamal", new_command.ensure_run_directory.join(" ")
   end
 
-  test "ensure non default run directory" do
-    assert_equal "mkdir -p /var/run/kamal", new_command(run_directory: "/var/run/kamal").ensure_run_directory.join(" ")
-  end
-
   private
     def new_command(extra_config = {})
       Kamal::Commands::Server.new(Kamal::Configuration.new(@config.merge(extra_config)))

test/configuration_test.rb

@@ -294,17 +294,11 @@ class ConfigurationTest < ActiveSupport::TestCase
   test "run directory" do
     config = Kamal::Configuration.new(@deploy)
     assert_equal ".kamal", config.run_directory
-
-    config = Kamal::Configuration.new(@deploy.merge!(run_directory: "/root/kamal"))
-    assert_equal "/root/kamal", config.run_directory
   end
 
   test "run directory as docker volume" do
     config = Kamal::Configuration.new(@deploy)
     assert_equal "$(pwd)/.kamal", config.run_directory_as_docker_volume
-
-    config = Kamal::Configuration.new(@deploy.merge!(run_directory: "/root/kamal"))
-    assert_equal "/root/kamal", config.run_directory_as_docker_volume
   end
 
   test "run id" do