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

Similar to https://codereview.webrtc.org/1539883002
but for GN. This was discovered during GN vs GYP auditing.

NOTRY=True
BUG=webrtc:6323

Review-Url: https://codereview.webrtc.org/2344633002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14220}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f807a521fa900b62b23978c19231552154d711a1

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