Remove DataSourceID and registration callback

Turns out we never actually need neither the notion of
a DataSourceID (!= DataSourceInstanceID) nor a callback
for RegisterDataSource. They represent only useless boilterplate.
It is just more convenient to use the data source name as a key,
we already rely on that being unique.
The proof is that none of the code we have made any use of that
and everything still works without.

Bug: 68854243
Test: perfetto_unittests / perfetto_integrationtests
Change-Id: I67ba8d3f5618760d530d56b474bd46e737484896
diff --git a/src/tracing/core/trace_writer_impl_unittest.cc b/src/tracing/core/trace_writer_impl_unittest.cc
index 5acf403..ad9156a 100644
--- a/src/tracing/core/trace_writer_impl_unittest.cc
+++ b/src/tracing/core/trace_writer_impl_unittest.cc
@@ -32,9 +32,8 @@
 namespace {
 
 class FakeProducerEndpoint : public Service::ProducerEndpoint {
-  void RegisterDataSource(const DataSourceDescriptor&,
-                          RegisterDataSourceCallback) override {}
-  void UnregisterDataSource(DataSourceID) override {}
+  void RegisterDataSource(const DataSourceDescriptor&) override {}
+  void UnregisterDataSource(const std::string&) override {}
   void CommitData(const CommitDataRequest&, CommitDataCallback) override {}
   SharedMemory* shared_memory() const override { return nullptr; }
   size_t shared_buffer_page_size_kb() const override { return 0; }