scripts: refactor remove structure forward declarations

Since now it has some problems when generate TAGS,
refactor this code. Now it will not show the error
message and will remove declarations using emacs etags.

Signed-off-by: Yang Bai <hamo.by@gmail.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>