mingw: get rid of getpass implementation

There's no remaining call-sites, and as pointed out in the
previous commit message, it's not quite ideal. So let's just
lose it.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -55,8 +55,6 @@ struct passwd {
 	char *pw_dir;
 };
 
-extern char *getpass(const char *prompt);
-
 typedef void (__cdecl *sig_handler_t)(int);
 struct sigaction {
 	sig_handler_t sa_handler;