Remove or consolidate theme functions and templates

There are many similar or related theme functions and template files that should be removed or consolidated. This here is a list of things we should do for Backdrop (that were either done in D8 or started)
Master list of all cleanup and notes is in this google doc.

Remove template files

• backdrop/backdrop#257 | Remove region.tpl.php
• #145 | Remove views-more.tpl.php (use theme_more_link instead)

Remove Theme functions

Form element containers

• backdrop/backdrop#324 | Remove theme_checkboxes (call theme_container instead) | d.o
• backdrop/backdrop#324 | Remove theme_radios (call theme_container instead) | d.o
• backdrop/backdrop#227 | Remove theme_system_compact_link() entirely. | d.o
• backdrop/backdrop#229 | Remove theme_book_title_link() use l() instead | d.o