Remove Enumerable include from Statement class (#1104)

I'm honestly not sure why this was here in the first place. Git blame
seems to suggest that maybe it was added when the class was first
created in anticipation for something that never materialized or has
since been deprecated.

As far as I can tell, `Enumerable` offers no advantages to the class
since `Statement` doesn't implement `#each`.

Here's an example of what I'm referring to
```ruby
client = Mysql2::Client.new
statement = client.prepare('select 1')
statement.first
statement.any?
statement.min
```

Also all test pass with `Enumerable` gone, so it seems reasonable to
remove it.

lib/mysql2/statement.rb

@@ -1,7 +1,5 @@
 module Mysql2
   class Statement
-    include Enumerable
-
     def execute(*args, **kwargs)
       Thread.handle_interrupt(::Mysql2::Util::TIMEOUT_ERROR_CLASS => :never) do
         _execute(*args, **kwargs)