signal/sig_dispatch: Revert changes that cause a regression

[pussuw]

Summary

This reverts changes that cause a regression in signal handling, more info in #8740

The problem still remains where signal actions are not performed / the user signal handler is not called if:

• The signal recipient is in syscall
• The signal recipient is waiting for a signal, i.e. stcb->task_state == TSTATE_WAIT_SIG

Only the process wake up part works, but the signal action is never performed. I need more time to fix this.

Impact

Fix regression

Testing

CI, code goes back to something that passes https://github.com/linux-test-project/ltp/blob/master/testcases/open_posix_testsuite/conformance/interfaces/timer_settime/9-2.c

[pussuw]

@xiaoxiang781216 I suggest we do the revert so I can take my time fixing the system call related issues