Delete deprecated pref declarations that no longer exist.

Bug: None
Change-Id: I56a7735ce113d7988d2e20f07d83efc3a602ad9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670876
Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672703}

components/metrics/metrics_pref_names.h

@@ -10,8 +10,6 @@ namespace prefs {
 
 // Alphabetical list of preference names specific to the metrics
 // component. Document each in the .cc file.
-extern const char kDeprecatedMetricsInitialLogs[];
-extern const char kDeprecatedMetricsOngoingLogs[];
 extern const char kInstallDate[];
 extern const char kMetricsClientID[];
 extern const char kMetricsDefaultOptIn[];