language_set_browser_drupal_langcode_mappings() is a useless wrapper, so remove it

Problem/Motivation

• In core, language_set_browser_drupal_langcode_mappings() is only called from one place, and that usage can be simplified to not call it.
• If we leave the function for contrib to use, we should add a test for it, so issues like #2392319: Config objects (but not config entities) should by default be immutable don't break it.
• Either removing the function, or deciding not to, is a blocker for #2392319: Config objects (but not config entities) should by default be immutable, so Critical priority.

Proposed resolution

Remove the function.

Remaining tasks

User interface changes

API changes

Remove language_set_browser_drupal_langcode_mappings()

Beta phase evaluation