Drop some redundant instructions in reduction in ecp_nistz256_sqr_montx.

(Imported from upstream's 8fc063dcc9668589fd95533d25932396d60987f9.)

I believe this is a no-op for us as we do not currently enable the
ADX-based optimizations.

Change-Id: I34a5f5ffb965d59c67f6b9f0ca7937e49ba6e820
Reviewed-on: https://boringssl-review.googlesource.com/16884
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>