SearchEngineTracker: Disable unstable switch P3A browser test.

Like the other SearchEngineTracker browser test, this gives unstable
results in ci, tracked in brave/brave-browser#13057.

Disable the test for now, and remove the `MergeHistogramDeltasForTesting`
which we don't expect is necessary and made no difference in testing.

browser/search_engines/search_engine_tracker_browsertest.cc

@@ -14,7 +14,6 @@
 #include "chrome/browser/ui/browser.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/search_test_utils.h"
-#include "components/metrics/content/subprocess_metrics_provider.h"
 #include "components/search_engines/template_url_prepopulate_data.h"
 #include "content/public/test/browser_test.h"
 
@@ -57,10 +56,7 @@ IN_PROC_BROWSER_TEST_F(SearchEngineProviderP3ATest,
 }
 
 IN_PROC_BROWSER_TEST_F(SearchEngineProviderP3ATest,
-                       SwitchSearchEngineP3A) {
-  // Make sure we're in sync.
-  metrics::SubprocessMetricsProvider::MergeHistogramDeltasForTesting();
-
+                       DISABLED_SwitchSearchEngineP3A) {
   // Check that the metric is reported on startup.
   // For some reason we record kNoSwitch twice, even through
   // kDefaultSearchEngineMetric is only updated once at this point.