Relanding: Remove the IceTransportInternal2.
Relanding: Remove the IceTransportInternal2.

Introduing IceTransportInternal2 is a temporary fix to switch the base
class of P2PTransportChannel to IceTransportInternal without breaking
Chromium. It is removed in this CL.

Reland this CL after Chromium doesn't depend on this.

BUG=webrtc:6951

Review-Url: https://codereview.webrtc.org/2632563002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16131}
Committed: https://chromium.googlesource.com/external/webrtc/+/c7953fa716dc26db1123bb80351d46e496792c81
Review-Url: https://codereview.webrtc.org/2632563002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16960}

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