Remove unnecessary declaration in win32_port.h

Mis-merge introduced by e2f21ff, where pgwin32_setenv() was listed but
not defined in win32env.c.  This had no consequences as this routine
does not exist in this branch.

Only REL_12_STABLE and REL_13_STABLE got that wrong.

Backpatch-through: 12

src/include/port/win32_port.h

@@ -462,7 +462,6 @@ extern void _dosmaperr(unsigned long);
 
 /* in port/win32env.c */
 extern int	pgwin32_putenv(const char *);
-extern int	pgwin32_setenv(const char *name, const char *value, int overwrite);
 extern void pgwin32_unsetenv(const char *);
 
 /* in port/win32security.c */
@@ -473,7 +472,6 @@ extern int	pgwin32_is_admin(void);
 extern BOOL AddUserToTokenDacl(HANDLE hToken);
 
 #define putenv(x) pgwin32_putenv(x)
-#define setenv(x,y,z) pgwin32_setenv(x,y,z)
 #define unsetenv(x) pgwin32_unsetenv(x)
 
 /* Things that exist in MinGW headers, but need to be added to MSVC */