dcsctp: Remove assignment of redundant metrics

The DcSctpSocket::GetMetrics() returns a Metrics struct that contains
both constant values calculated when the connection was established,
such as capabilities, and also dynamic information that change for each
incoming or outgoing packet.

The constant values are stored within "metrics_" when the TCB is created
and copied into the returned metrics during GetMetrics() call. The
dynamic values are then only set on the local copy before returning.

Before this CL, the "negotiated_maximum_incoming_streams" metric, which
is a constant metric, was additionally set - twice - in GetMetrics(),
which could make one believe that the
"negotiated_maximum_outgoing_streams" was never set at all, but it was,
and this was also verified with the
DcSctpSocketParametrizedTest::ExposesTheNumberOfNegotiatedStreams test
case.

This change removes the redundant assignment, but please note that the
returned Metrics have always been correct.

Bug: webrtc:487768780
Change-Id: If29f5c28dc5499bb9dac8b461bdc15462671b9b1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/452700
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47011}