use explicit arguments for mysql

scripts/remove

@@ -18,6 +18,7 @@ app=$YNH_APP_INSTANCE_NAME
 domain=$(ynh_app_setting_get $app domain)
 port=$(ynh_app_setting_get $app port)
 db_name=$(ynh_app_setting_get $app db_name)
+db_user=$db_name
 final_path=$(ynh_app_setting_get $app final_path)
 
 #=================================================
@@ -51,7 +52,7 @@ ynh_remove_app_dependencies
 #=================================================
 
 # Remove a database if it exists, along with the associated user
-ynh_mysql_remove_db $db_name $db_name
+ynh_mysql_remove_db $db_user $db_name
 
 #=================================================
 # REMOVE APP MAIN DIR