x86, signals: remove duplicated register setup code in ia32 signal delivery

Impact: cleanup, no functionality changed

ia32_setup_rt_frame() has a duplicated code block labelled
"Make -mregparm=3 work" for setting up the register parameters
to the user-mode signal handler.

This is harmless but ugly. Remove the redundant assignments.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Ingo Molnar <mingo@elte.hu>