Remove now-unnecessary dirty form checks on save/preview shortcuts

Save is handled by the 'is this a form submission' check, and preview was probably
never needed at all (preview opens in a new window and does not unload the page,
unless there's a validation error, which is handled separately)

wagtail/wagtailadmin/static_src/wagtailadmin/js/page-editor.js

@@ -333,13 +333,11 @@ function initCollapsibleBlocks() {
 
 function initKeyboardShortcuts() {
     Mousetrap.bind(['mod+p'], function(e) {
-        disableDirtyFormCheck();
         $('.action-preview').trigger('click');
         return false;
     });
 
     Mousetrap.bind(['mod+s'], function(e) {
-        disableDirtyFormCheck();
         $('.action-save').trigger('click');
         return false;
     });