Drop run_directory configuration option

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

lib/kamal/configuration.rb

@@ -202,15 +202,11 @@ class Kamal::Configuration
 
 
   def run_directory
-    raw_config.run_directory || ".kamal"
+    ".kamal"
   end
 
   def run_directory_as_docker_volume
-    if Pathname.new(run_directory).absolute?
-      run_directory
-    else
-      File.join "$(pwd)", run_directory
-    end
+    File.join "$(pwd)", run_directory
   end
 
   def hooks_path