Remove unnecessary dependency on system_wrappers in libjingle.

The system_wrappers target is already included due to the dependency
on the <(DEPTH)/third_party/webrtc/webrtc.gyp:webrtc target.

Review URL: https://codereview.chromium.org/851873004

Cr-Commit-Position: refs/heads/master@{#311455}

third_party/libjingle/libjingle.gyp

@@ -593,7 +593,6 @@
             '<(libjingle_source)/talk/media/webrtc/webrtcvoiceengine.h',
           ],
           'dependencies': [
-            '<(DEPTH)/third_party/webrtc/system_wrappers/source/system_wrappers.gyp:system_wrappers',
             '<(DEPTH)/third_party/webrtc/voice_engine/voice_engine.gyp:voice_engine',
             '<(DEPTH)/third_party/webrtc/webrtc.gyp:webrtc',
             '<@(libjingle_peerconnection_additional_deps)',