Delete old default_socket.h header

The header was moved from ext/ to tracing/ in aosp/2821944.
Now that the old header is no longer used in Chromium (after
crrev.com/c/5067679 is submitted), we can delete the old file.

Also fix a comment in include/perfetto/tracing/tracing.h.

Bug: b/245116106
Change-Id: I8048c0dee2ec5378fc3a5d00487bf3d9dd05b5c3

test/test_helper.cc

diff --git a/test/test_helper.cc b/test/test_helper.cc
index a576874..c9da9c2 100644
--- a/test/test_helper.cc
+++ b/test/test_helper.cc
@@ -20,8 +20,8 @@
 #include "perfetto/base/compiler.h"
 #include "perfetto/ext/base/string_utils.h"
 #include "perfetto/ext/tracing/core/trace_packet.h"
-#include "perfetto/ext/tracing/ipc/default_socket.h"
 #include "perfetto/tracing/core/tracing_service_state.h"
+#include "perfetto/tracing/default_socket.h"
 
 #include "protos/perfetto/trace/trace_packet.pbzero.h"