Removing WEBRTC_BUILD_LIBEVENT macro definition

This seems to be unused in WebRTC and a search in cs.chromium.orm shows
that it is never used in chromium too.

Interesting CL about this macro: https://codereview.webrtc.org/2153563003

BUG=None

Review-Url: https://codereview.webrtc.org/3006243002
Cr-Original-Commit-Position: refs/heads/master@{#19747}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 63f968e0cef08a88b711e121d61baa86f64fa0b5