Revert "Let PrintResultMeanAndError accept a string."

This reverts commit b54bc06079a66a379595068f6ea56d56a4fd95c0.

Reason for revert: Downstream projects should be fixed now

Original change's description:
> Let PrintResultMeanAndError accept a string.
> 
> Some downstream projects still use it. I'll update them and then revert
> this change.
> 
> Bug: webrtc:8566
> Change-Id: Ib4e56348c40a3645f3049382b47089ca6c675e96
> Reviewed-on: https://webrtc-review.googlesource.com/25841
> Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
> Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20872}

TBR=phoglund@webrtc.org,ehmaldonado@webrtc.org

Change-Id: I5d2a59cbfb6a148fc6c621a69fa23397ba2c6991
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:8566
Reviewed-on: https://webrtc-review.googlesource.com/25920
Reviewed-by: Edward Lemur <ehmaldonado@webrtc.org>
Commit-Queue: Edward Lemur <ehmaldonado@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20880}
diff --git a/test/testsupport/perf_test.cc b/test/testsupport/perf_test.cc
index cf244df..bb4491a 100644
--- a/test/testsupport/perf_test.cc
+++ b/test/testsupport/perf_test.cc
@@ -85,16 +85,6 @@
                    units, important);
 }
 
-void PrintResultMeanAndError(const std::string& measurement,
-                             const std::string& modifier,
-                             const std::string& trace,
-                             const std::string& mean_and_error,
-                             const std::string& units,
-                             bool important) {
-  PrintResultsImpl(measurement, modifier, trace, mean_and_error, "{", "}",
-                   units, important);
-}
-
 void PrintResultList(const std::string& measurement,
                      const std::string& modifier,
                      const std::string& trace,
diff --git a/test/testsupport/perf_test.h b/test/testsupport/perf_test.h
index 2c98b8e..02dd1f3 100644
--- a/test/testsupport/perf_test.h
+++ b/test/testsupport/perf_test.h
@@ -52,13 +52,6 @@
                              const std::string& units,
                              bool important);
 
-void PrintResultMeanAndError(const std::string& measurement,
-                             const std::string& modifier,
-                             const std::string& trace,
-                             const std::string& mean_and_error,
-                             const std::string& units,
-                             bool important);
-
 // Like PrintResult(), but prints an entire list of results. The |values|
 // will generally be a list of comma-separated numbers. A typical
 // post-processing step might produce plots of their mean and standard