Remove features from Social Private Message module

Problem

See problem definition in #3092272: [Meta] Moving away from features. The fact that this configuration is in a feature means that login help text gets overwritten.

Solution

• Remove the social_private_message.features.yml file file
• Move the default editable config from hook_install() into a config/install/*.yml file.
• Remove any _module_set_defaults function

Steps to reproduce

• Enable the module
• Enable email notifications
• Revert features