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: refs/heads/master@{#11089}

webrtc/modules/audio_coding/codecs/isac/isacfix.gypi

@@ -77,11 +77,6 @@
         'fix/source/structs.h',
       ],
       'conditions': [
-        ['OS!="win"', {
-          'defines': [
-            'WEBRTC_LINUX',
-          ],
-        }],
         ['target_arch=="arm" and arm_version>=7', {
           'sources': [
             'fix/source/lattice_armv7.S',