Remove the watchdog's fd tracker.

The existence of both fdtrack and the older OpenFdMonitor is confusing
to partners, and not obviously useful because the fdtrack limit is so
much lower than the OpenFdMonitor limit (now that the default
RLIMIT_NOFILE is 32Ki).

Bug: https://issuetracker.google.com/161199049
Test: builds
Change-Id: I86436f287d0e25d1dfae95b915beb78c88dccce2