Remove install configuration

fwiw, removing the default config isn't needed here, on a clean composer install + drush enable, on Drupal 8.0.3

	composer require drupal/smtp:dev-8.x-1.x
	cat config/install/smtp.settings.yml
		smtp_on: false
		smtp_host: ''
		smtp_hostbackup: ''
		smtp_port: ''
		smtp_protocol: ''
		smtp_username: ''
		smtp_password: ''
		smtp_from: ''
		smtp_fromname: ''
		smtp_allowhtml: ''
		smtp_test_address: ''
		smtp_debugging: false

	cat smtp.install
		...
		/**
		 * Implements hook_install().
		 */
		function smtp_install() {
		  // Setting default config vars.
		  \Drupal::service('config.factory')->getEditable('smtp.settings')
		    ->set('smtp_on', 0)
		    ->set('smtp_host', '')
		    ->set('smtp_hostbackup', '')
		    ->set('smtp_port', '25')
		    ->set('smtp_protocol', 'standard')
		    ->set('smtp_username', '')
		    ->set('smtp_password', '')
		    ->set('smtp_from', '')
		    ->set('smtp_fromname', '')
		    ->set('smtp_allowhtml', '')
		    ->set('smtp_test_address', '')
		    ->set('smtp_debugging', 0)
		    ->save();
		}
		...

@
BUG: Uninstall fails with ImmutableConfigException
https://www.drupal.org/node/2653338#comment-10848258

	wget https://www.drupal.org/files/issues/smtp-uninstall_fails_with-2653338-4.patch
	patch -p1 < smtp-uninstall_fails_with-2653338-4.patch

@
BUG: Invalid placeholder: !url" errors when accessing the config page
https://www.drupal.org/node/2611210

	wget https://www.drupal.org/files/issues/smtp-show_link_in_configuration_page-2611210-3-D8.patch
	patch -p1 < smtp-show_link_in_configuration_page-2611210-3-D8.patch

then

	drush -y pmu smtp
	drush cr
	drush -y en smtp

	drush cget smtp.settings
		smtp_on: false
		smtp_host: ''
		smtp_hostbackup: ''
		smtp_port: '25'
		smtp_protocol: standard
		smtp_username: ''
		smtp_password: ''
		smtp_from: ''
		smtp_fromname: ''
		smtp_allowhtml: ''
		smtp_test_address: ''
		smtp_debugging: false
		_core:
		  default_config_hash: X...X

With no errors in the install.

Settings can now be toggled at will, either at cli, e.g.

drush -y cset smtp.settings smtp_on true
drush -y cset smtp.settings smtp_on false

or in the UI.