Revert "Generate and use the real key to sign UMA/UKM uploads."

This reverts commit 867eeca264.

Reason for revert: Cause of https://crbug.com/930891

Original change's description:
> Generate and use the real key to sign UMA/UKM uploads.
> 
> BUG=906202
> 
> Change-Id: I10d2de8a1f30c0a56cc95f69aa6c252f171dca73
> Reviewed-on: https://chromium-review.googlesource.com/c/1461089
> Commit-Queue: Jesse Doherty <jwd@chromium.org>
> Reviewed-by: Roger Tawa <rogerta@chromium.org>
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#630939}

TBR=rogerta@chromium.org,jwd@chromium.org,rkaplow@chromium.org

Change-Id: If85da5416f52f414942ca8d75e5f5b4611a86842
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 906202
Reviewed-on: https://chromium-review.googlesource.com/c/1465066
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630956}

google_apis/google_api_keys.h

@@ -79,9 +79,6 @@ std::string GetNonStableAPIKey();
 void SetAPIKey(const std::string& api_key);
 #endif
 
-// Retrieves the key used to sign metrics (UMA/UKM) uploads.
-std::string GetMetricsKey();
-
 // Represents the different sets of client IDs and secrets in use.
 enum OAuth2Client {
   CLIENT_MAIN,         // Several different features use this.