Remove redefinitions of SIG_* macros in win32_port.h.

It is not clear why these were originally added.  One hypothesis is
that an ancient version of MinGW didn't define them.  In any case,
they appear to now be superfluous, so let's remove them.  If
nothing else, the buildfarm might offer us clues to their origins.

Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/Z4chOKfnthRH71mw%40nathan

Author: nathan-bossart

src/include/port/win32_port.h

@@ -154,14 +154,6 @@
 
 #define sigmask(sig) ( 1 << ((sig)-1) )
 
-/* Signal function return values */
-#undef SIG_DFL
-#undef SIG_ERR
-#undef SIG_IGN
-#define SIG_DFL ((pqsigfunc)0)
-#define SIG_ERR ((pqsigfunc)-1)
-#define SIG_IGN ((pqsigfunc)1)
-
 /* Some extra signals */
 #define SIGHUP				1
 #define SIGQUIT				3