Remove webrtc_all target

Move all its configuration to all.gyp instead, which is
not processed by Chromium builds (webrtc.gyp is).

BUG=webrtc:5949
NOTRY=True

Review-Url: https://codereview.webrtc.org/2060873002
Cr-Original-Commit-Position: refs/heads/master@{#13124}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 86600246ca357d9c92a13c3bd431e0b2f3aab08d