Drop mysql2 support, settle on postgresql

It's enough work to support one database, let's settle on
the defacto Rails default.

lib/tasks/dump_db.rake

@@ -1,13 +0,0 @@
-# frozen_string_literal: true
-
-require 'yaml'
-task :dump_db do
-  yaml = YAML.load_file("config/database.yml")
-  conf = yaml["production"]
-  filename = "#{conf["database"]}-#{Time.now.strftime("%Y-%m-%d-%H:%M:%S:%L")}.sql"
-  if conf["adapter"] == 'mysql2'
-    system "mysqldump -u #{conf["username"]} --password=#{conf["password"]} -h #{conf["host"]} #{conf["database"]} > ~/#{filename}"
-  else
-    puts "Error: This rake task only works for MYSQL"
-  end
-end