Remove the IceTransportInternal2.
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.

BUG=webrtc:6951

Review-Url: https://codereview.webrtc.org/2632563002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16083}

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