iSAC: Remove unnecessary WEBRTC_LINUX define.
iSAC: Remove unnecessary WEBRTC_LINUX define.

I can only find one use in iSAC codebase:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/webrtc/modules/audio_coding/test/iSACTest.cc&l=19

It's the prime suspect for causing a compilation error for iOS failing to
include linux/net.h which is being included in
webrtc/voice_engine/voice_engine_defines.h

NOTRY=True

Review URL: https://codereview.webrtc.org/1539883002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#11089}

TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc