Removed unused forward declaration.
Removed unused forward declaration.

TBR=kwiberg@webrtc.org
NOTRY=True
BUG=webrtc:6346

Review-Url: https://codereview.webrtc.org/2491483002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14999}

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