Fix -Wunused-but-set-variable compiler warnings in tevent_signal.c

Fix -Wunused-but-set-variable compiler warnings in tevent_signal.c

The results of some read(2) and write(2) calls are assigned into a
variable that is never used.  Presumably this used to avoid compiler
warnings or similar.

However, from (approximately) GCC 4.6 this produces some warnings:

  [ 609/3910] Compiling lib/tevent/tevent_signal.c
  ../lib/tevent/tevent_signal.c: In function ‘tevent_common_signal_handler’:
  ../lib/tevent/tevent_signal.c:85:10: warning: variable ‘res’ set but not used [-Wunused-but-set-variable]
  ../lib/tevent/tevent_signal.c: In function ‘signal_pipe_handler’:
  ../lib/tevent/tevent_signal.c:183:10: warning: variable ‘res’ set but not used [-Wunused-but-set-variable]

The simplest thing to do is remove the variables and cast the function
return to void.  There is already a comment above each call.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Signed-off-by: Stefan Metzmacher <metze@samba.org>