x86: ia32.h: remove unused struct sigfram32 and rt_sigframe32

Impact: cleanup

Remove struct sigfram32 and rt_sigframe32 because there is no user.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>