Remove 'diff' from s:filetype_overrides

I would like to see the normal statusline (i.e. file name) with
diff/patch files, too.

autoload/airline/extensions.vim

@@ -22,7 +22,6 @@ let s:script_path = tolower(resolve(expand('<sfile>:p:h')))
 let s:filetype_overrides = {
       \ 'nerdtree': [ 'NERD', '' ],
       \ 'gundo': [ 'Gundo', '' ],
-      \ 'diff': [ 'diff', '' ],
       \ 'vimfiler': [ 'vimfiler', '%{vimfiler#get_status_string()}' ],
       \ 'minibufexpl': [ 'MiniBufExplorer', '' ],
       \ 'startify': [ 'startify', '' ],