Remove dead code for detecting system SRTP

We currently don't support unbundling SRTP because Chromium uses a too
new unreleased version, but we were still testing for it and claiming to
use the system one if found.

Change-Id: I250f0d7fc1e09398b3196895bdd529d366cd0a86
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>