[identifiability] Remove IdentifiabilityStudyState

As part of cleaning up the Identifiability Study / Privacy Budget
code, this CL removes the IdentifiabilityStudyState together with the
code that instantiated and owned it in UKM infrastructure.

Bug: 451513551
Change-Id: I04e498dd487aae2ab66d4dd560634ba3d4065159
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7209046
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1552142}
NOKEYCHECK=True
GitOrigin-RevId: 0d71dc34984b70f2d997b1023f5a824d93945737

metrics_service_client.cc

diff --git a/metrics_service_client.cc b/metrics_service_client.cc
index 3bb36bb..d991157 100644
--- a/metrics_service_client.cc
+++ b/metrics_service_client.cc
@@ -100,11 +100,6 @@
   return nullptr;
 }
 
-IdentifiabilityStudyState*
-MetricsServiceClient::GetIdentifiabilityStudyState() {
-  return nullptr;
-}
-
 structured::StructuredMetricsService*
 MetricsServiceClient::GetStructuredMetricsService() {
   return nullptr;

metrics_service_client.h

diff --git a/metrics_service_client.h b/metrics_service_client.h
index 1808323..5464d89 100644
--- a/metrics_service_client.h
+++ b/metrics_service_client.h
@@ -40,8 +40,6 @@
 class SyntheticTrialRegistry;
 }
 
-class IdentifiabilityStudyState;
-
 namespace metrics {
 
 class MetricsLogUploader;
@@ -81,10 +79,6 @@
   // Returns the PumaService instance that this client is associated with.
   virtual metrics::private_metrics::PumaService* GetPumaService();
 
-  // Returns the IdentifiabilityStudyState instance that this client is
-  // associated with. Might be nullptr.
-  virtual IdentifiabilityStudyState* GetIdentifiabilityStudyState();
-
   // Returns the StructuredMetricsService instance that this client is
   // associated with.
   virtual structured::StructuredMetricsService* GetStructuredMetricsService();