Remove use_system_libsrtp build knob.

As of https://crbug.com/328475, libsrtp in Chromium is now built against
BoringSSL's crypto primitives rather than using libsrtp's crypto
implementation. Any library that is expected to build against a bundled
library (there is no such thing as system BoringSSL, and Chromium does
not support any BoringSSL version skew) fundamentally cannot be unbundled.
In this case, the problem manifested itself as a failure to run things
inside the sandbox.

We don't support use_system_* options anyway, but as use_system_libsrtp
no longer works and won't ever work again, remove it.

BUG=501318
R=jiayl@chromium.org, phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/1662363002 .
2 files changed