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
#xx | Remove book-all-books-block.tpl.php
#145 | Remove views-more.tpl.php (use theme_more_link instead)
#N/A | Remove book-export-html.tpl.php See: #242
#N/A | Remove book-node-export-html.tpl.php See: #242

HOLD OFF ON THESE FOR NOW

#xx | Remove user-picture.tpl.php Related: #243

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
#xx | Remove theme_date (call theme_container instead)
#xx | Remove theme_textfield (add theme_input and call it instead)
#xx | Remove theme_button (call theme_input instead)
#xx | Remove theme_checkbox (call theme_input instead)
#xx | Remove theme_email (call theme_input instead)
#xx | Remove theme_file (call theme_input instead)
#xx | Remove theme_hidden (call theme_input instead)
#xx | Remove theme_image_button (call theme_input instead)
#xx | Remove theme_image_button (call theme_input instead)
#xx | Remove theme_password (call theme_input instead)
#xx | Remove theme_radio (call theme_input instead)
#xx | Remove theme_submit (call theme_input instead)
#xx | Remove theme_tel (call theme_input instead)
backdrop/backdrop#227 | Remove theme_system_compact_link() entirely. | d.o
backdrop/backdrop#229 | Remove theme_book_title_link() use l() instead | d.o

item lists

#xx | Remove theme_links() and call theme('item_list__links') instead. | d.o
#xx | Remove theme_task_list() and call theme('item_list__tasks') instead. | d.o
#xx | Remove all specific menu link theme functions, use theme_menu() with suggestions instead. | d.o

tables

#xx | Remove theme_image_style_list() and call theme('table__image_styles') instead. | d.o
#xx | Remove theme_image_style_effects() and call theme('table__image_styles__effects') instead. | d.o
#xx | Remove theme_book_admin_table and call theme('table__book_admin') instead. | d.o