Remove custom tracing in net::disk_cache

This CL is removing the old tracing system in net::disk_cache.
The tracing is only active in debug build, or if manually toggle
an ifdef in trace.cc (see #define ENABLE_TRACING 0)

R=gab@chromium.org, morlovich@chromium.org

Bug: 1065004
Change-Id: I8d0b85975b15896078d9275fd9ab6ac6174338e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122449
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753776}