[Instrumented libraries] Drop Trusty support

This CL removes all platform-specific logic in
//third_party/instrumented_libraries.  The rationale is that I don't
expect to have separate versions of the instrumented libraries, besides
when upgrading. For the future Xenial->Bionic upgrade (which I plan on
working on soon since standard support for Xenial ended April 2021, and
our infrastructure is already on Bionic) I plan to copy
//third_party/instrumented_libraries and remove the old one after the
upgrade.

R=thestig

Change-Id: I24468e5b33b97240ccf73c8f843c0d4f84a39259
Bug: 1244199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3195029
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#926572}