Remove support for S/Key

Most people will 1) be using modern multi-factor authentication methods
like TOTP/OATH etc and 2) be getting support for multi-factor
authentication via PAM or BSD Auth.
diff --git a/auth2-chall.c b/auth2-chall.c
index 4fd18f4..2d5cff4 100644
--- a/auth2-chall.c
+++ b/auth2-chall.c
@@ -58,9 +58,6 @@
 #ifdef USE_PAM
 extern KbdintDevice sshpam_device;
 #endif
-#ifdef SKEY
-extern KbdintDevice skey_device;
-#endif
 #endif
 
 KbdintDevice *devices[] = {
@@ -70,9 +67,6 @@
 #ifdef USE_PAM
 	&sshpam_device,
 #endif
-#ifdef SKEY
-	&skey_device,
-#endif
 #endif
 	NULL
 };
@@ -369,7 +363,7 @@
 void
 privsep_challenge_enable(void)
 {
-#if defined(BSD_AUTH) || defined(USE_PAM) || defined(SKEY)
+#if defined(BSD_AUTH) || defined(USE_PAM)
 	int n = 0;
 #endif
 #ifdef BSD_AUTH
@@ -378,9 +372,6 @@
 #ifdef USE_PAM
 	extern KbdintDevice mm_sshpam_device;
 #endif
-#ifdef SKEY
-	extern KbdintDevice mm_skey_device;
-#endif
 
 #ifdef BSD_AUTH
 	devices[n++] = &mm_bsdauth_device;
@@ -388,8 +379,5 @@
 #ifdef USE_PAM
 	devices[n++] = &mm_sshpam_device;
 #endif
-#ifdef SKEY
-	devices[n++] = &mm_skey_device;
-#endif
 #endif
 }