pgo_tools: remove mention of llvm-next_pgo_use

This flag was removed in crrev.com/c/4907700

BUG=b:306906485
TEST=Unittests

Change-Id: I346d560f71a679e7d93226d7e5a8c47a6f1c0398
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/5007840
Auto-Submit: George Burgess <gbiv@chromium.org>
Commit-Queue: Jordan Abrahams-Whitehead <ajordanr@google.com>
Reviewed-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed