Revert "[CQ.cfg] 50% of chromium_presubmit builds will now run on LUCI."

This reverts commit 38feaec8.

Reason for revert: commit-git-patch flakily fails on LUCI, e.g. https://ci.chromium.org/p/chromium/builders/luci.chromium.try/chromium_presubmit/21881

Original change's description:
> [CQ.cfg] 50% of chromium_presubmit builds will now run on LUCI.
> 
> R=​dpranke@chromium.org, jbudorick@chromium.org
> 
> No-Try: True
> Bug: 731553
> Change-Id: I12cd97bfee4d1cc7bf81f5c990014a27335a1c50
> Reviewed-on: https://chromium-review.googlesource.com/891666
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#532539}

TBR=dpranke@chromium.org,tandrii@chromium.org,jbudorick@chromium.org

Change-Id: I5ea0844f22c10335ffb0de5a360d76a792442781
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 731553
Reviewed-on: https://chromium-review.googlesource.com/893418Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532858}

infra/config/cq.cfg

@@ -78,7 +78,7 @@ verifiers {
       }
       builders {
         name: "chromium_presubmit"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 50 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
       }
       builders {
         name: "fuchsia_arm64"