Remove unnecessary comparison.

Popped up after https://reviews.llvm.org/D142826 added extra flags to
-Wextra, which is used by our sanitizer buildbots
(https://lab.llvm.org/buildbot/#/builders/37/builds/19910).

This check seems unnecessary, it's a bad cargo-cult after the buffer
size was expanded to allow >= 4GiB after
https://reviews.llvm.org/rGd6c15b661ab0aabb00f1219ce4af7136938e67e2.