Remove unnecessary code from psql's watch command

Hi,

In the current code of do_watch(), sigsetjmp is called if WIN32
is defined, but siglongjmp is not called in the signal handler
in this condition. On Windows, currently, cancellation is checked
only by cancel_pressed, and  calling sigsetjmp in do_watch() is
unnecessary. Therefore, we can remove code around sigsetjmp in
do_watch(). I've attached the patch for this fix.

Regards,
Yugo Ngata

-- 
Yugo NAGATA <nagata@sraoss.co.jp>