DEV: Switch off of our image_optim fork (#13124)

The main image_optim gem now includes the timeout feature
that we had in our fork. So it is now safe to switch off of our fork and
back to the image_optim gem.

This is the link to the commit in the image_optim repo that adds the
timeout option:

ec3767dde0

One difference with the new timeout implementation is that image_optim
now handles the timeout exceptions instead of bubbling them up:

1ed0328587/lib/image_optim.rb (L128-L129)

```
 rescue Errors::TimeoutExceeded
   handler.result
```

So a timeout will just return `nil`, which is the same response if it
couldn't optimize an image. I don't think we were really watching for
or doing anything about these timeout warnings in our logs so I think
this is an okay change to have and we will have less warnings in our
logs now too.

Gemfile

@@ -90,9 +90,7 @@ gem 'unf', require: false
 
 gem 'email_reply_trimmer'
 
-# Forked until https://github.com/toy/image_optim/pull/162 is merged
-# https://github.com/discourse/image_optim
-gem 'discourse_image_optim', require: 'image_optim'
+gem 'image_optim'
 gem 'multi_json'
 gem 'mustache'
 gem 'nokogiri'