runtime: avoid new linkname for goroutine profiles
CL 464349 added a new linkname to provide gcount to runtime/pprof to
avoid a STW when estimating the goroutine profile allocation size.
However, adding a linkname here isn't necessary for a few reasons:
1. We already export gcount via NumGoroutines. I completely forgot about
this during review.
2. aktau suggested that goroutineProfileWithLabelsConcurrent return
gcount as a fast path estimate when the input is empty.
The second point keeps the code cleaner overall, so I've done that.
For #54014.
Change-Id: I6cb0811a769c805e269b55774cdd43509854078e
Reviewed-on: https://go-review.googlesource.com/c/go/+/559515
Auto-Submit: Michael Pratt <mpratt@google.com>
Auto-Submit: Nicolas Hillegeer <aktau@google.com>
Reviewed-by: Nicolas Hillegeer <aktau@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>