runtime: remove unused skip arg from fpTracebackPCs

runtime: remove unused skip arg from fpTracebackPCs

This was accidentally left behind when moving the logic to set the skip
sentinel in pcBuf to the caller.

Change-Id: Id7565f6ea4df6b32cf18b99c700bca322998d182
Reviewed-on: https://go-review.googlesource.com/c/go/+/489095
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>