Remove zstd from third_party/.gitignore.

third_party/zstd has been replaced with third_party/zstd-linux-arm64
and third_party/zstd-linux-x64.

Bug: 1154022
Change-Id: I87c58d5bbf1bf57a5a64c03255e4731cd1df0eb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611897Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Stephen Roe <steveroe@google.com>
Cr-Commit-Position: refs/heads/master@{#840644}

third_party/.gitignore

@@ -262,6 +262,5 @@
 /xdg-utils
 /xstream/lib/
 /xulrunner-sdk
-/zstd
 /zstd-linux-arm64
 /zstd-linux-x64