kex.c: removed dupe entry from libssh2_kex_methods[]

Closes #51

src/kex.c

@@ -1711,7 +1711,6 @@ static const LIBSSH2_KEX_METHOD *libssh2_kex_methods[] = {
     &kex_method_diffie_helman_group_exchange_sha256,
     &kex_method_diffie_helman_group_exchange_sha1,
     &kex_method_diffie_helman_group14_sha1,
-    &kex_method_diffie_helman_group_exchange_sha1,
     &kex_method_diffie_helman_group1_sha1,
     NULL
 };