Remove unneeded code from template_preprocess_field()

This issue has novice tasks. If you are an experienced core developer and have multiple commit mentions, please review novices' work on these tasks rather than doing them yourself. Feedback from experienced contributors is valued.

Problem/Motivation

Now that theme_field() is gone (or will be gone for good after #2226233: Redo changes from field.module to Twig conversion issue that were clobbered is complete) we can remove some unneeded code from template_preprocess_field():

  // The default theme implementation for fields is a function.
  // template_preprocess() (which initializes the attributes, title_attributes,
  // and content_attributes arrays) does not run for theme function
  // implementations. Additionally, Attribute objects for the three variables
  // below only get instantiated for template file implementations, and we need
  // Attribute objects for printing in both theme functions and template files.
  // For best performance, we only instantiate Attribute objects when needed.
  $variables['attributes'] = isset($variables['attributes']) ? new Attribute($variables['attributes']) : clone $default_attributes;
  $variables['title_attributes'] = isset($variables['title_attributes']) ? new Attribute($variables['title_attributes']) : clone($default_attributes);
  $variables['content_attributes'] = isset($variables['content_attributes']) ? new Attribute($variables['content_attributes']) : clone($default_attributes);

Proposed resolution

Remove the code.

Remaining tasks

• Write patch (novice)
• Review patch to check it fixes the issue, the change is properly documented and for coding standards. Provide test evidence (novice)
• Keep issue summary up to date (novice)

User interface changes

n/a

API changes

n/a