fix(linux): Remove wrong `ok_for_single_backspace` method

The implementation of `ok_for_single_backspace` checked the current
action_item multiple times. However, we already know that that
item is `KM_KBP_IT_BACK` because the only place this method gets
called is from the `process_backspace_action` method. So basically
this method always returns `FALSE`. "Fixing" this method to check
the next action_items makes things fail.

Removing this method since it doesn't do anything except burn
processing cycles...

Fixes #5614.