Delete forward declarations from peer_connection_interface.h
Delete forward declarations from peer_connection_interface.h

Bug: None
Change-Id: I011b5c8ae81055ae5b4941438af226665dcbd075
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152825
Reviewed-by: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Reviewed-by: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Commit-Queue: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29191}

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