Also removes listing of targets in webrtc_fuzzers which is very prone to
not being up to date. They're not required for ClusterFuzz integration
or building locally. This also means that adding fuzzers won't require
approval outside the fuzzers directory.
BUG=webrtc:4771
R=kjellander@webrtc.org, stefan@webrtc.org
Review URL: https://codereview.webrtc.org/1518973003 .
Cr-Commit-Position: refs/heads/master@{#11067}
