themes: drop simple colorscheme - theme mappings

vim-airline by default already tries to load theme names, that have the
same name as the colorscheme. Therefore, it does not make sense, to add
mappings that result in the same theme name.

So drop the base16, wombat, zenburn and solarized keys

autoload/airline/init.vim

@@ -59,11 +59,7 @@ function! airline#init#bootstrap()
   call extend(g:airline_theme_map, {
         \ 'default': 'dark',
         \ '\CTomorrow': 'tomorrow',
-        \ 'base16': 'base16',
         \ 'mo[l|n]okai': 'molokai',
-        \ 'wombat': 'wombat',
-        \ 'zenburn': 'zenburn',
-        \ 'solarized': 'solarized',
         \ 'flattened': 'solarized',
         \ '\CNeoSolarized': 'solarized',
         \ }, 'keep')