runtime: remove residual !go115NewMCentralImpl fields
runtime: remove residual !go115NewMCentralImpl fields

Change-Id: I1685721c82be4ac3c854084592e5e0f182b367ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/266858
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/go,nawawi/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go