removed tw-timer.h/c

- used only by epi, tcp-ip, tcp, and timer models
removed periodic pe function

- removed g_tw_periodicity variable
- removed pe_periodic_f type
- removed periodic function call in tw-sched.c (conservative)
removed tw_register_signals (unused)

- removed commented call
- removed extern decl
- removed code file (signal-sigaction.c)