Fix some unreachable code in curve25519_64_adx.h's fiat_addcarryx_u64

This codepath would only be used on MSVC, and this file isn't used on
MSVC, but still fix it before this code gets copy-pasted somewhere else.

Change-Id: Ic81f1cb09978890ea20b1046562a36428dcca4f0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/93987
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>

third_party/fiat/curve25519_64_adx.h

diff --git a/third_party/fiat/curve25519_64_adx.h b/third_party/fiat/curve25519_64_adx.h
index 30e9ae9..cdabd2e 100644
--- a/third_party/fiat/curve25519_64_adx.h
+++ b/third_party/fiat/curve25519_64_adx.h
@@ -85,9 +85,7 @@
   *out2 = addcarry64(arg1, arg2, arg3, &t);
   *out1 = t;
 #elif defined(_M_X64)
-  long long unsigned int t;
   *out2 = _addcarry_u64(arg1, arg2, arg3, out1);
-  *out1 = t;
 #else
   arg2 += arg1;
   arg1 = arg2 < arg1;