Remove SBClientDownload.DownloadFileHasDmgSignature

This metric is expired and not expected to be useful in the near future.

Fixed: 1037229
Change-Id: I2e83b9f4cd8b652c4b9b4c0f8f70e216a3b96bfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2238965
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Bettina Dea <bdea@chromium.org>
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Bettina Dea <bdea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776802}

chrome/browser/safe_browsing/download_protection/check_client_download_request_base.cc

@@ -532,11 +532,6 @@ void CheckClientDownloadRequestBase::SendRequest() {
   request->set_download_type(type_);
 
 #if defined(OS_MACOSX)
-  UMA_HISTOGRAM_BOOLEAN(
-      "SBClientDownload."
-      "DownloadFileHasDmgSignature",
-      disk_image_signature_ != nullptr);
-
   if (disk_image_signature_) {
     request->set_udif_code_signature(disk_image_signature_->data(),
                                      disk_image_signature_->size());