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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#11089}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 740c367af39845dd79e190d57cfe9f6faa8ae3e5

Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk